For search engines that crawl the web, links are the streets between pages. Using sophisticated link analysis, the engines can discover how pages are related to each other and in what ways.

Since the late 1990's search engines have used links as 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 complex algorithms create nuance evaluations of sites and pages based on this information.

Links aren't everything in SEO, but search professionals attribute a large portion of the engines' algorithms to link-based factors (see Search Engine Ranking Factors). Through links, engines can not only analyze the popularity of a website & page based on the number and popularity of pages linking to them, but also metrics like trust, spam, and authority. Trustworthy sites tend to link to other trusted sites, while spammy sites receive very few links from trusted sources (see mozTrust). Authority models, like those postulated in the Hilltop Algorithm, suggest that links are a very good way of identifying expert documents on a given subject.

Promotional Signs

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.

 

used by 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. Search engines use links in many different ways. While we don't know all the link attributes measured by the engines, 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.

Global Popularity

The more popular and important a site is, the more links from that site matter. A site like Wikipedia has literally 1000's 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.

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 matter more than links from general or off-topic sites. For example, if your website sells dog houses, earning links from the Society of Dog Breeders matters much more than earning links from an off-topic, roller skating site.

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", where many results rank solely due to the anchor text of inbound links.

TrustRank

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.

Link Neighborhood

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 the totality of these links in aggregate, search engines can understand the "link neighborhood" your website exists in. 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.

Freshness

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 not only to earn links to your website, but also to continue to earn additional links over time. Commonly referred to as "FreshRank," search engines use the freshness signals of links to judge current popularity and relevance.

Social Sharing

The last few years has 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 algorithm, but there is no denying the rising importance of social channels.

 

 

The Concept of Trustrank