A primary use of browsers is to search the Web. If you know the Web address of a site you want to access, you can enter it in the address line of the browser. Sometimes, however, you need to search by general topic or by one or more keywords. When you click a command such as Search the Web in your browser, it takes you to a search engine.
Search engines generally consist of two types:

1. Categories (such as Yahoo and AltaVista) that focus on a broad range of topics for you to choose from. Most Category engines require the Webmaster to register their site with the engine by informing the engine of the site's domain name, the particular category, and even keywords most appropriate for that site's reference.
   
2. Indexes (such as Lycos, Excite, and Infoseek) which use Web crawlers (previously called spiders) that read the Web pages of the Internet and index their keywords for reference.

Web crawlers and spiders continuously explore the Web, reading all documents and storing their keywords in a large database.
To use a category search engine, you progress through a sequence of categories pertinent to their search. For example, Aircraft, Propulsion Systems, Propellers. Presently, even category engines permit users to locate sites by their keywords.
When using an indexed engine, the user enters one or more keywords to identify the site most appropriate to their objectives: propellers. A more complex search can entail many keywords, such as, propellers, biplane, 1918. To facilitate the great array users, most commercial search engines try to provide both keyword and category search techniques. In fact, the latest trend is to narrow a keyword search by first selecting the general subject area.
An example of a search engine is illustrated below: