Search engines and their indexes evaluate far more than just the words you write.
In addition to your words, search engine indexes contain “metadata” (data about data). Metadata can consist of markup (intentional metadata – like alt. and title tags), measurements, counts, summaries, positions, and pointers.
The first search engines used minimal metadata. Most depended on text matching, an artifact of their origins as simple content databases.
When modern search engines first emerged, these systems used algorithms that measured how pages and sites *related* to each other.
One very simple way to measure popularity was to count the number of “links in”, links pointed *to* a page. Then, it was reasonable to assume that a page with over 100 “links in”, was more popular than a page with very few “links in”.
As a result of this simple scoring, the number of sites linking to a page played a major role in how that page ranked. For any given keyword or phrase, the site receiving the most links were usually at the top of the search results.
Not All Links Are Created Equal
In the early days, search engines treated every link like a vote for the site receiving the link. Each link raised a page’s rank by the same amount. The problem with this democratic approach? Many began to stuff the ballot boxes.
Bad webmasters became SEO spammers, creating large networks of web sites containing nothing but advertising, keywords, and links to each other. Every link acted like a fraudulent vote. Driving up the ranking of the sites under their control. This tactic was very effective when first introduced.
However, search engine developers soon realized that these “link farms” made it much harder for users to find the information they wanted. Link farms Hijacked search engine results, landing visitors on sites filled with nothing but advertising.
To counter, search engines stopped treating every link as a vote and assigned each link a score based in part on which site was doing the linking. Search engines now use several factors when determining how much weight to assign to a given link, but the most important is the “credibility” of the link source.
Measuring Link Quality
Today, the rules have changed. Search engines are no longer simply counting links; they’re paying attention to the source of the link. Link building in the age of Search Engine Optimization puts the emphasis on link quality as opposed to quantity.
Seeking links which will float your site with a higher ranking is as important to good SEO as the content of your site. Even more important? Know which links will sink your content.
Credible Site Links Count for More
Do you cook? When you try a new recipe found on the internet, does it come from a random blogger? Or, are you more likely to try a recipe found on a familiar and popular cooking site?
The difference is credibility. A credible website is trustworthy and demonstrates expertise. At the very least, the people running the cooking site know what they’re doing and are more likely to offer a tasty recipe with clear instructions.
Search engines now look at links with this idea of credibility in mind. A link on a popular, high-quality site is better than the same link found on an unknown blogger’s pages. In fact, one link from a trusted source is worth dozens of links from random, low-traffic sites.
Untrusted Sites Can Hurt
In an attempt to thwart link fraud, search engines have learned how to spot links coming from dubious sites.
If a linking site contains almost no original content, but many links to other sites, each link found there receives very little voting power. In fact, if a site receives too many links from these sources, the search engines will begin to penalize it.
Again, it comes down to trust. If your site only ever receives links from untrusted sources and paid link directories, the search engines will assume your site has little or no value to users.
Pay close attention to where you go looking for links.
For maximum SEO leverage, look for sites which are highly-ranked for your chosen keywords and topics and seek links from them.
Take the time to be selective. Make sure you are doing more good than harm.