How search engines work

The term search engine is often used generically to describe both true search engines and directories. They are not the same. The difference is how they compile the results listings.

Search engines, such as HotBot, create their listings automatically. Search engines crawl the web, then people search through what they have found.

Search engines have three major elements. First is the spider or the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. The spider returns to the site on a regular basis, such as every month or two, to look for changes.

Everything the spider finds goes into the second part of a search engine: the index. The index, or catalogue, contains a copy of every web page that the spider finds. If a web page changes, then this book is updated with the new information.

Sometimes it can take a while for new pages or changes that the spider finds to be added to the index. So until the page added to the index, it is not available to those searching with the search engine.

The Search engine software is the third part of a search engine. This is the program that sifts through the millions of pages in the index to find matches to a search and to rank them in order of what it believes is most relevant.

All search engines have the basic parts, but there are differences in how these parts are tuned. That is why the same search on different search engines often produces different results.

Directories, such as Yahoo, depend on humans for its listings. You submit a short description to the directory for your entire site, or editors write one for sites they review. A search looks for matches only in the descriptions submitted.

Some search engines also maintain an associated directory.