For an excellent summary of the other systems, and how best to utilize them, please see TerraCom Inc.'s excellent explanation of Internet Resource Search Tools. The WWW receives the most attention because of the attractive and dynamic way it presents information. How we search is dependent upon where we are searching. Each system provides its own search tools. Search Engines are basically gigantic databases of text.
Some engines catalog key words while others attempt to catalog either concepts or the full text.
Because of the size of these engines, they are incredibly powerful. This is both a blessing and a curse. However, if we cannot be precise in our search, the engines will return hundreds of documents which may or may not be of interest. Search Indices are typically organized around categories such as company name, product or service type, or geographic region. Most indices are hierarchal, meaning that they begin with very broad categories.