Search engines view links as votes for one’s site but also understand that not all links have equal value. A link from a top ranked web site for example would carry more weight than a link from a site ranked 1 million. The process of determining the quality of links is a very complex one. Here are a few things a search engine may look at:

The relevancy of the site linking to one’s site 

Logic says that if one has an excellent site about bicycles, other sites within the bicycle industry would link to it. This is the exact reason one should never be afraid to ask for a link from a competitor. If the competitor is any good they will know the importance of links, if not, one can explain it to them.

 Always remember that one want to get as many links from within one’s industry as possible in order to establish a solid foundation. One can furthermore not really be penalized for incoming links. Links from other industries will simply carry less weight or in the worst case have a weight of zero.

The PR value of the page linking to one’s site. 

The PR score of a page linking to one is important, the higher the better. This however does not carry as much weight by itself as it once did. Simply download the Google Toolbar to view the PR of any site. This provides a good yardstick for the sites value. One must however also keep the items below in mind and not only base the power of a link on its PR value. 

The frequency at which links are added/edited/deleted on the page linking to one’s site. 

A search engine may monitor how a site uses its linking power. What this means is they will keep track of how many new links are added, the frequency at which they are added and how often they are deleted or replaced. They do this in order to get a better idea of sites link stability. This helps determine if a site is actually a "good site" or one that is merely selling links. Sites that sell links will logically be more active in the add/edit/delete categories.

 Search engines give more value to old links that have "aged" than to new fresh ones. Links that one have had to one’s site for 2 years would logically be more valuable than ones one have had for 1 week. This is why new links are put in a "sandbox" and not fully counted for 6-9 months. Keep this in mind if one ever wish to purchase links, one will need to commit to at least 1 year. 

The IP address of the site linking to one’s site 

Location of a link is important. A site that is linked to "naturally" will have links from all over the net. They will not only have links from sites on the same "address" as them. That does not seem natural. How can a site only have links from one place and expect to be truly popular? To win an "election" one need links or votes from all over.

 Another reason the IP address is important is due to the fact that webmasters would build huge networks of interlinked sites.

The ownership details of the site linking to one’s site 

Again in order to combat link networks search engines may look at the "whois" info of a site. This way they can determine if several different people are linking to a site, or if it's all the same person. Should one wish to view the "whois" info for any site, one can simply visit www.whois.sc

Should a top ranking site develop "organically" logic tells us it would have many links from several different sites each with different owners