These spiders continuously roam over the pages of information that comprise the World Wide Web, taking keywords and pieces of data from each page they crawl. This collected data is then added to a searchable index. When you use a search engine you are basically hoping for a match between the keywords you input and those the spider has placed in the index. The following is a selected list of search engines. No one search engine is an index of the entire World Wide Web.
Some indexes are larger than others, but even the largest has indexed only a portion of the web .
See the Search Engine Size Comparison Chart at Search Engine Watch for more information. The way that a search engine's spider operates will affect the index. These factors will determine the size and scope of the index. Web pages appear and disappear from the World Wide Web on a daily basis. Similarly, it may take some time for new pages placed on the World Wide Web to be added to the index.