There were many search engines before Google. Largely, they worked by crawling the Web and listing the terms found in each page, in an inverted index. An inverted index is a data structure that makes it easy, given a term, to find (pointers to) all the places where that term occurs.

When a search query (list of terms) was issued, the pages with those terms were extracted from the inverted index and ranked in a way that reflected the use of the terms within the page. Thus, presence of a term in a header of the page made the page more relevant than would the presence of the term in ordinary text, and large numbers of occurrences of the term would add to the assumed relevance of the page for the search query.

As people began to use search engines to find their way around the Web, unethical people saw the opportunity to fool search engines into leading people to their page. Thus, if you were selling shirts on the Web, all you cared about was that people would see your page, regardless of what they were looking for. Thus, you could add a term like “movie” to your page, and do it thousands of times, so a search engine would think you were a terribly important page about movies. When a user issued a search query with the term “movie,” the search engine would list your page first. To prevent the thousands of occurrences of “movie” from appearing on your page, you could give it the same color as the background. And if simply adding “movie” to your page didn’t do the trick, then you could go to the search engine, give it the query “movie,” and see what page did come back as the first choice. Then, copy that page into your own, again using the background color to make it invisible.

Techniques for fooling search engines into believing your page is about something it is not, are called term spam.The ability of term spammers to operate so easily rendered early search engines almost useless. To combat term spam, Google introduced Page Rank