Search engines rank web pages by the amount of in-bound links they have.  A web page with 2 in-bound links will rank better than a similar web page with 1 in-bound link.  Simple enough?  It would be if it stopped there.

A link can be worth less if it comes from a web page that has little or nothing to do with the web page that it is linking to.  Further more, some sites have higher rankings than others (i.e., more inbound quality links) so a link from a web page with a high ranking will be worth more than a link from a web page with a low ranking.

Also, a recpricated link, or a two-way link exchange, will not be worth as much as a one-way link.  Search engines have come to understand what web masters do to game search engines and so give less weight to links that look unnatural.

It is very difficult to get other sites to link to yours naturally.  Many large companies hire content writers whose sole purpose is to create amazing content that other web masters and bloggers will be compelled to link to.  This is a great way to build an authority in the search engines but it can be difficult to do with smaller, niche sites.  Many of us work on five or more websites at a time and do not want to focus to much time on a particular niche site.  This means that we need to find other ways to build links to our sites.

There are a lot of different types of links that can point to your web pages and you cannot control those links all the time.  Search engines know this and so do not expect all of your one-way inbound links to be perfectly optimized for the page they are pointing to.  So, if you have 500 links to a web page and they all have the same anchor text, you will not look natural.  It is much more natural for a site to have a large variety of links pointing to it that are not-optimized links.

As described in the lessons at The Keyword Academy, it is  very important for all of your inbound links to look authentic and a great way to do that is by having a variety of links pointing to your web pages with different anchor text.  Mark and Court over at the Keyword Academy actually recommend only using your targeted keyword as the exact anchor text 25% of the time.  The rest should be varied, even to the point of not being optimized.  Do this better than your competition and you will out rank them in the search engines.