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.

“Sybil attack” 

This is the forging of multiple identities for malicious intent, named after the famous multiple personality disorder patient Shirley Ardell Mason. 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.

Wiki spam 

Using the open editability of wiki systems to place links from the wiki site to the spam site. Often, the subject of the spam site is totally unrelated to the page on the wiki where the link is added. In early 2005, Wikipedia implemented a ‘nofollow’ value for the ‘rel’ HTML attribute. Links with this attribute are ignored by Google’s PageRank algorithm. Forum and Wiki admins can use these to end or discourage Wiki spam.

Spam in blogs 

This is the placing or solicitation of links randomly on other sites, placing a desired keyword into the hyperlinked text of the inbound link. Guest books, forums, blogs and any site that accepts visitors comments are particular targets and are often victims of drive by spamming where automated software creates nonsense posts with links that are usually irrelevant and unwanted.

Spam blogs 

Also known as splogs, a spam blog, on the contrary, is a fake blog created exclusively with the intent of 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.

Referer log spamming 

When someone accesses a web page, i.e. the referee, by following a link from another web page, i.e. the referer, the referee is given the address of the referer by the person’s internet browser. Some websites have a referer log which shows which pages link to that site. By having a robot randomly access many sites enough times, with a message or specific address given as the referer, that message or internet address then appears in the referer log of those sites that have referer logs. Since some search engines base the importance of sites by the number of different sites linking to them, referer-log spam may be used to increase the search engine rankings of the spammer’s sites, by getting the referer logs of many sites to link to them.

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.

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.