Link spamming is a form of black hat SEO that tries to exploit weaknesses in link-based ranking algorithms such as the one used by Google’s PageRank algorithm. PageRank give a higher ranking to a website that is linked to many other websites so the idea is to add as many links as possible to a page to try and boost your page rankings.
There are several forms of link spamming. An easy one is called link farming which involves creating communities of pages that refer to each other in order to bulk up a link index.
Another tactic is called the “Sybil attack” where you put up links to unrelated but popular sites where visitors will not see them. This is the forging of multiple identities for malicious intent, named after the famous woman with the multiple personality disorder. 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. Another type is called Wiki spam, which uses the open editing platform of wiki systems to place links on a page.
Yet another popular technique is Splogging also known as spam blogging. This is the placing of links randomly on other sites in blogs, forums, guest books and forums.
Page hijacking is so common it is ridiculous. Web hosts do it when you forget to renew your domain name. Page hijacking 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. Some link spammers monitor DNS records for domains that will expire soon, and then buy them when they expire and replace the pages with links to their pages.
Link Spamming is classified as a high risk way to do effective SEO, you risk having your domain banned by Google if you do.