What is link spam?

Link spam takes advantage of link-based ranking algorithms, such as Google's PageRank algorithm, which gives a higher ranking to a website the more other highly ranked websites link to it. These techniques also aim at influencing other link-based ranking techniques such as the HITS algorithm.

Link farms 

Involves creating tightly-knit communities of pages referencing each other, also known humorously as mutual admiration societies.

Hidden links 

Putting links where visitors will not see them in order to increase link popularity. Highlighted link text can help rank a webpage higher for matching that phrase.

"Sybil attack" 

This is the forging of multiple identities for malicious intent, named after the famous multiple personality disorder patient "Sybil". A spammer may create multiple web sites at different domain names that all link to each other, such as fake blogs known as spam blogs.

Spam blogs 

Spam blogs, also known as splogs, are fake blogs created solely for spamming. They are similar in nature to link farms.

Page hijacking 

This is achieved by creating a rogue copy of a popular website which shows contents similar to the original to a web crawler but redirects web surfers to unrelated or malicious websites.

Buying expired domains 

Some link spammers monitor DNS records for domains that will expire soon, then buy them when they expire and replace the pages with links to their pages. However Google resets the link data on expired domains.

Some of these techniques may be applied for creating a Google bomb, this is, to cooperate with other users to boost the ranking of a particular page for a particular query.