Yahoo has over 1 million sites listed. Yahoo also supplements its results with those from Inktomi. If a search fails to find a match within Yahoo's own listings, then matches from Inktomi are displayed. Inktomi matches also appear after all Yahoo matches have first been shown. Yahoo is the oldest major web site directory, having launched in late 1994.
Search engines crawl the web, then people search through what they have found.
If you change your web pages, search engines eventually find these changes, and that can affect how they are found. Page titles, body copy, and other elements all play a role. A directory such as Yahoo depends on humans for its listings. You submit a short description to the directory for your site, or editors write ones for the sites they review. A search looks for matches only in the descriptions submitted.