Unsavory web sites owners on the Internet, with their constant need to generate meaningless traffic, have spammed the search engines over the years. They have ranked their web sites for terms that have no real relevance by optimize pages for popular terms and submitting them.
In order to combat this, most search engines (especially Google) have added another major component to their algorithm: Relevancy.
Because of the spammers, search engines can no longer trust a web site's relevancy just by the Meta-tags and words within the content of the page. They have figured into their search equation, that if your web site is truly about 'Subject A,' you would logically have other web sites about 'Subject A' linking to you.
Search engines want to provide the most meaningful search results to their users querying them every day. As part of their relevancy requirement, a web site that has many industry related links must be a better resource than a web site in the same industry with no links.
It is in this regard, that Google instituted their web site Page Rank. As stated on their web site, it is a measure of the importance of a certain page within a web site. Each page in a web site has a different Page Rank. The scoring system is 0-10; ten being the best, 0 being the worst.
Web sites with a higher page rank tend to rank better for relevant terms than their competitors. An important page, in Google's algorithm, is one with incoming links from other trusted sources on the web.
Incoming links from other web sites must be performed correctly in order to generate added relevancy to your site. The three biggest components of a good link are the actual link text, the page the link is coming from and the page the link is pointing to. All three have to maintain a direct keyword consistency.
If the link is coming from a page with a low Page Rank (or worse a dynamic, unspiderable page) or the link text and keywords in the destination page do not match up, the link will not have any perceived relevancy to the search engines. In fact, it may water down your overall link popularity.
A link is best performed by someone who can evaluate the potential link partner, spend time to follow through with the link exchange and check to make sure the correct link code was used.
