I'm currently working on re-authoring and re-building the Beginner's Guide to Search Engine Optimization, section by section. You can read more about this project here. 

For search engines that crawl the web, links are the connectors between pages. Using link analysis, the engines can discover how pages are related to other pages and in what ways. Since the late 1990's links have also served as a stand-in for votes - representing the democracy of the web's opinion about what pages are important and popular. The engines themselves have refined the use of link data to a fine art, and incredibly sophisticated algorithms create nuanced evaluations of site and pages based on this information.

Professional SEOs attribute a considerable portion of the search engines' algorithms to link-based factors (see Search Engine Ranking Factors). Through links, engines analyze the popularity of a site & page based on the number and popularity of pages linking to them, as well as metrics like trust, spam, & authority. More trustworthy sites tend to link to other trusted sites, while spammy sites receive very few links from trusted sources (see TrustRank). Authority models, like those postulated in the Hilltop Algorithm, suggest that links are a very good way of identifying expert documents in a given space. 

Thanks to this focus on algorithmic use and analysis of links, growing the link profile of a website is critical to gaining traction, attention, and traffic from the engines. As an SEO, link building is among the top tasks required for search ranking and traffic success.

Link Building Basics

Building links is an art. It's almost certainly the most challenging part of an SEO's job, and, for many sites, the one most critical to achieving long term success. Many companies can afford to hire SEOs to help make their websites search friendly and search optimized, but a robust backlink profile is an extremely high barrier to competition.

There are essentially three types of link acquisition:

• Editorial Accumulation - Links that are given naturally by sites and pages that want to reference your content or company. These links require no specific action from the SEO, other than the creation of citation-worthy material and the ability to create awareness about it to relevant communities.
• Manual Suggestion & Approval - Emailing bloggers with links, submitting sites to directories, or paying for listings of any kind fit into this group. The SEO must create a value proposition with the link target and complete that transaction manually (whether it be filling our forms for submissions to a website award program or convincing a professor that your resource is worthy of inclusion on the public syllabus).
• Self-Created, Non-Editorial - Hundreds of thousands of websites offer any visitor the opportunity to create links through guestbook signings, forum signatures, blog comments, or user profiles. These links are typically quite low in value, but can, in aggregate, have a significant impact. However, automatic methods of generating these links is certainly spamming, and even the manual creation of such links is frowned upon by many site owners and search engines. Exceptions abound, and for those sites that offer these options and don't use the rel="nofollow" attribute on outbound links, there can be opportunity.

It's up to you, as an SEO, to select which of these will have the highest return on the effort invested. As a general rule, it's wise to build as vast and varied a link profile as possible, as this brings the best search engine results. Any link building pattern that appears non-standard, unnatural, or manipulative will eventually become a target for advancing search algorithms to discount.

Link Signals Used by the Search Engines

Before embarking on a link building effort, it's critical to understand the elements of a link used by the search engines as well as how those elements factor into the weighting of links in the algorithms. We don't know all the attributes measured by the engines, but through analysis of patent applications, papers submitted to IR conferences, and hands-on experience & testing, we can draw some intelligent assumptions. Below is a list of notable factors worthy of consideration:

• Global Popularity - the more popular and important a site is, the more links from that site matter to the search engines. Getting lots of local, topic-specific links is great, too, but to earn trust and authority with the engines, you'll need the help of some powerful link partners.
• Local/Topic-Specific Popularity - the concept of "local" popularity (first pioneered by the Teoma search engine) suggests that links from sites within a topic-specific community matters more than links from general or off-topic sites.
• Anchor Text - One of the strongest signals the engines use in rankings is anchor text. If dozens of links point to a page with the right keywords, that page has a very good probability of ranking well for the targeted phrase in that anchor text. You can see examples of this in action with searches like "click here" and "leave," where many results rank solely due to the anchor text of inbound links.
• TrustRank - In order to weed out massive amounts of spam (some estimate as much as 60% of the web's pages are spam), search engines use systems for measuring trust, many of which are based on the link graph. Earning links from highly trusted domains can, thus, result in a significant boost to this scoring metric.
• Link Neighborhood - In many papers on spam detection and information retrieval, using the sites that link to a domain as well as the sites that domain links to has an impressive knack for spam detection and filtering. Thus, it's wise to choose those sites you link to carefully and be equally selective with the sites you attempt to earn links from.

All of these issues, and many more, are considered by professional SEOs when measuring link value and a site's link profile.

Starting a Link Building Campaign

As with any marketing activity, the first process undertaken in a link building campaign must be the creation of goals, along with a set of strategies to reach those goals. Sadly, link building is one of the most difficult activities to measure, particularly from a search engine optimization perspective. Although the engines internally weight each link with precise, mathematical metrics, it's impossible for those outside of the engineering teams at these companies to extract this data. Thus, as SEOs, we rely on a number of signals to help build a rating scale of link value. Along with the less-measurable data from the link signals mentioned above, these metrics include:

• Page Ranking for Relevant Search Terms - One of the best ways to determine how well a search engine values a given page is to search for some of the keywords and phrases that page targets (particularly those in the title and headline). Pages that rank well for relevant queries tend to be more valuable than those that don't.
• Google PageRank - Despite much maligning over the years for accuracy and freshness problems (Google only updates their toolbar PageRank data every 3-5 months and sometimes manipulates the values intentionally to discourage spam and overanalysis), there is still value to looking at the number reported. Pages with high PageRank do tend to pass on more link value than those with little or none. Be careful with those that have PageRank "unranked" ( a gray bar) as these may be highly valuable pages that simply haven't received visible PageRank since the last update.
• Domain Popularity & Authority - Though a tough metric to judge with hard numbers, sources like Alexa, Compete, Quantcast, and Google Trends for Websites can all be of assistance in assessing the relative popularity of a website. More popular sites tend to be more valuable for link acquisition.
• Yahoo! Site Explorer Reported Inlinks - Yahoo! Site Explorer is a valuable tool for seeing the links that point to a given site or page. Using this tool, you can make estimates about the relative link popularity and importance a page has based on who links to it. Typically those pages/sites with more powerful and important links will pass on greater value through their links.
• Google Blogsearch Reported Inlinks - Google Blog Search is the only property controlled by the search giant that offers accurate backlink information (the link: search at google.com is virtually useless, and doesn't even present a representative quantity of link numbers). While this only shows links from blogs and feeds, there's still great value in seeing which sites/pages have earned authority and attention in the blogosphere, as this can be a useful predictor of the link value they'll pass.
• Number of Links on a Page - According to the original PageRank formula (and confirmed recently by testing), the value that a link passes is diluted by the presence of other links on a page. Thus, getting linked-to by a page with few links is better than being linked-to by the same page with many links on it (all other things being equal). The degree to which this is relevant is unknowable (and in our testing, it appears to be important, but not overwhelmingly so), but it's certainly something to be aware of as you conduct link acquisition.
• Potential Referral Traffic - Link building should never be solely about search engines. Links that send high amounts of direct click-through traffic not only tend to provide better search engine value for rankings, but also send targeted, valuable visitors to your site (the basic goal of all Internet marketing). This is something you can estimate based on the numbers of visits/page views according to site stats, but if you can't get access to these, services like Google Trends for Websites, Compete, Quantcast, & Alexa can give you a rough idea of at least domain-wide traffic, from which you can estimate page-specific popularity.
  

It takes time, practice, and experience to build comfort with these variables as they relate to search engine traffic. However, using your website's analytics (at SEOmoz, we love Indextools, but there's a lengthy list of other good tools here), you should be able to determine whether your campaign is successful. Increases in search traffic when accompanied by more frequent search engine crawling and increases in referring link traffic correlates with a well-managed, intelligently structured campaign. If you see traffic from engines like MSN/Live and Yahoo! are rising while Google stays constant, it's possible that you need to seek more authoritative, better trusted links (as Google is the most "picky" of the engines when it comes to link evaluation).  

The link building activities you engage in depend largely on the type of site you're working with - for smaller sites, manual link building, including directories, link requests, and link exchanges may be a part of the equation, but with larger sites, these tactics tend to fall flat and more scalable solutions are required. Sample strategies are listed below, though this is by no means an exhaustive list (see SEOmoz's Professional's Guide to Link Building for a more comprehensive overview).

Five Sample Link Building Strategies

Search for sites like yours in the search engines by using keywords and phrases directly relevant to your business. When you locate sites that aren't directly competitive, you can email them, use their online forms, call them on the phone, or even send them a letter by mail to start a conversation about getting a link. Check out this blog post on email link requests for more detail.

1. Many sites offer directories or listings of relevant resources. You can find hundreds of these on SEOmoz's Directory List or use the search engines themselves to find lists of pages that offer outbound links in this fashion (for example, try searching for allintitle: resources directory at Google and notice the millions of results). For more detail see this post on what makes a good directory and this one on the role of directories in link building.
2. Get your customers to link to you. If you have partners you work with regularly or loyal customers that love your brand, you can use this to your advantage by sending out partnership badges - graphic icons that link back to your site (like Microsoft often does with their partner certification program). Just as you'd get customers wearing your t-shirts or sporting your bumper stickers, links are the best way to accomplish the same feat on the web. Check out this post on link requests in order confirmation emails for more.
3. Build a company blog and make it a valuable, informative and entertaining resource. This content and link building strategy is so popular and valuable that it's one of the few recommended personally by the engineers at Google (source: USA Today & Stone Temple). Blogs have the unique ability to contribute fresh material on a consistent basis, participate in conversations across the web, and earn listings and links from other blogs, including blogrolls and blog directories.
4. Create content that inspires viral sharing and natural linking. In the SEO world, we often call this "linkbait." Good examples might include this Peak Season Ingredient Map from Epicurious, this Interactive Graphic Explaining Hand Signals Used on Stock Market Trading Floors from the New York Times, or this Video of an iPod in a Blender from Blendtec. Each leverages aspects of usefulness, information dissemination, or humor to create a viral effect - users who see it once want to share it with friends, and bloggers/tech-savvy webmasters who see it will often do so through links. This high quality, editorially earned votes are invaluable to building trust, authority, and rankings potential.
5. Build content that can be shared through a citation-based licensing agreement. If you have photos, videos, graphics, charts, raw data, or text content that can be licensed out with a system like Creative Commons' Attribution (or Attribution-ShareAlike), you can leverage the power of the web's penchant for information sharing while receiving links back to your originals and your site each time someone uses your material.

An Aside on Buying Links

Buying links, although it can be an important part of many campaigns, is viewed negatively by the search engines. Google, Yahoo!, and MSN/Live all seek to discount the influence of paid links on their search results, but this applies to certain kinds of purchased links. In general, the engines' opposition targets links that are bought and sold primarily for the purposes of boosting search engine rankings. Thus, they typically exclude examples like Yahoo! Directory (which requires a tough editorial evaluation) or Business.com listings (which also pass through an editorial process).

Strong negative action is taken, particularly by Google, against sites that engage in buying or selling links through link brokers (that don't use robots controls to indicate non-editorial links like the rel=nofollow tag or a javascript redirect). However, at SEOmoz, we still will recommend paid links of some kinds, and many professional SEOs and websites endorse and see value in link buying. For a more thorough evaluation of the subject, see blog posts on The Quizzical Duality of Paid Links and The Art of Buying Links Under the Radar.

Obviously, this project is taking me far longer than anticipated, but with only a few sections to go, I'm hopeful that it will be completed soon, at which point I can combine the posts into a guide, edit, add more illustrations, and launch. As always, comments and criticisms (particularly constructive ones) are heartily welcomed.