Most major search engines consider a link a vote of confidence.
A link is a connection to another position on the same webpage, or a referral from another website to yours. Link building is done to improve the reach, visibility and accessibility of your content. The idea here is that each link counts as a vote towards the trustworthiness of your website. Each link your website has establishes your website as authoritative and trustworthy. However, not all links are weighted equally. The easiest way to explain this is to use the concept of block voting. Block voting is when one person is given the authority to vote on behalf of many. This is the way unions work. For example:
  »  Frank represent 100 people
  »  Julie represents 200 people and
  »  Joe represents 500 people

All three decide to vote on an issue that affects their represented groups, Julie and Frank vote against the motion, but Joe votes for the motion. It may seem that Joe loses in a 2 against 1 vote, but because Joe represents 500 people, he technically has 500 votes against Frank and Julie’s 300 votes. Joe wins. All websites are given a value of PageRank and this counts towards their authority. Some websites that you link to will have more authority than others, and the higher ranked sites will give your website a higher vote. The most important factor in establishing link quality is the trustworthiness of the website linking to yours. If you have more trusted and valuable websites linking to yours, the more trusted and valuable your website becomes. All major search engines have a link-based rating model. Most of them look at:
  »  Anchor text
  »  The semantic attributes of the text around your link and
  »  The relationship between linked sites.

These ranking systems are much harder to manipulate than the other systems that measure organic ranking. Here search engines look at IP addresses, reciprocal links and domain registration. This is an attempt to determine if your page is valuable, or if you are engaging in link farming. The perfect link should:
   1. Send valuable traffic to your website
   2. Tell the search engines your site is an authority.