How Search Engines Fight Link Spam?

[senthil]

How spammers try to circumvent search engine guidelines to obtain high rankings for sites that may not deserve those rankings. Of course, the search engines do many things to fight link spam. please someone share own thought to understand about this.

[Fli]

I do not know, but i imagine it like this:
they set certain rules, example:

1)
if they find 30 times one link with same anchor text and with same text around this link on several sites (lets call them link farms). And at same time they find the link farms contains similar bulk/unnatural links + low quality content which has low number of natural visitors

2)
then they may think this is some junk site (link farm) made just for backlinking and someone is attempting to manipulate site ranking placing their backinks there in bulk

3)
so they add less weight to the site, especially if they find content quality of the destination/promoted site is very low or contains signs of an attempt to manipulate site ranking by using excessive amount of keyword phrasses or such.