Search engines are essentially massive full-text indexes of web pages. The quality of the indexes, and how the engines use the information they contain, is what makes or breaks the quality of search results. We're all familiar with indexes in the back of book. They're simply alphabetical lists of the important words in the book, and the pages on which they appear. Search engine indexes are similar, but vastly more complex than back of the book indexes. Indexing is carried out by search engine robots, crawlers (spiders) and on directories by humans.