Used by search engines
How do search engines assign value to links? To answer this, we need to explore the individual elements of a link, and look at how the search engines assess these elements. We don't fully understand the proprietary metrics that search engines use, but through analysis of patent applications, years of experience, and hands-on testing, we can draw some intelligent assumptions that hold up in the real world. Below is a list of notable factors worthy of consideration. These signals, and many more, are considered by professional SEOs when measuring link value and a site's link profile.
The more popular and important a site is, the more links from that site matter. A site like Wikipedia has thousands of diverse sites linking to it, which means it's probably a popular and important site. To earn trust and authority with the engines, you'll need the help of other link partners. The more popular, the better.
The concept of "local" popularity, first pioneered by the Teoma search engine, suggests that links from sites within a topic-specific community matter more than links from general or off-topic sites. For example, if your website sells dog houses, a link from the Society of Dog Breeders matters much more than one from a site about roller skating.
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," where many results rank solely due to the anchor text of inbound links.
It's no surprise that the Internet contains massive amounts of spam. Some estimate as much as 60% of the web's pages are spam. In order to weed out this irrelevant content, search engines use systems for measuring trust, many of which are based on the link graph. Earning links from highly-trusted domains can result in a significant boost to this scoring metric. Universities, government websites and non-profit organizations represent examples of high-trust domains.
Spam links often go both ways. A website that links to spam is likely spam itself, and in turn often has many spam sites linking back to it. By looking at these links in the aggregate, search engines can understand the "link neighborhood" in which your website exists. 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.
Link signals tend to decay over time. Sites that were once popular often go stale, and eventually fail to earn new links. Thus, it's important to continue earning additional links over time. Commonly referred to as "FreshRank," search engines use the freshness signals of links to judge current popularity and relevance.
The last few years have seen an explosion in the amount of content shared through social services such as Facebook, Twitter, and Google+. Although search engines treat socially shared links differently than other types of links, they notice them nonetheless. There is much debate among search professionals as to how exactly search engines factor social link signals into their algorithms, but there is no denying the rising importance of social channels.