People who do this are called search engine spammers. The word is a portmanteau of spamming and indexing (as well as a pun on spandex.)

Search engines use a variety of algorithms to determine relevancy ranking. Some of these include determining whether the search term appears in the META keywords tag, others whether the search term appears in the body text of a web page. A variety of techniques are used to spamdex, including listing chosen keywords on a page in small-point font face the same colour as the page background (rendering it invisible to humans but not search engine web crawlers).

Search engine spammers are generally aware that the content that they promote is not very useful or relevant to the ordinary internet surfer. They try to use methods that will make the website appear above more relevant websites in the search engine listings. The rise of spamdexing in the mid-1990s made the leading search engines of the time less useful, and the success of Google at both producing better search results and combating keyword spamming, through its reputation-based PageRank link analysis system, led directly to its becoming the dominant search site late in the decade, where it remains. However, while it has not been rendered useless by spamdexing, Google has not been immune to more sophisticated methods either. Spamdexing on Google has generated the term Google bombing.

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