What Are Databases?

A database is a collection of pieces of related information that is organized so its various informational items can be located and retrieved when needed. Billions of electronic databases exist, but only some of them can be accessed on the Web. Your health insurer, for example, probably has a searchable database that contains information about your recent health history, but that database is not available on the Web.

On the other hand, many databases are indeed available on the Web. Some databases on the Web can be used by anyone at no charge, and others require that users belong to a particular organization or pay a fee. Web-accessible databases are part of what is sometimes referred to as the hidden Web, the deep Web, or the invisible Web: information that cannot be directly located with today's general-purpose search engines because it is "hidden" behind query forms. To locate information in most web-accessible databases, you need to be able to find the database itself and make your query. Databases on the Web include library catalogs, telephone books, public records, and news and magazine archives. For example, if you use a web search engine to look for a specific book, you probably won't find Amazon.corn's page about the book because Amazon's book information is stored in a database that isn't indexed by search engines.

Some databases are fully indexed, meaning that you can search the entire database for the occurrence of any word. Other databases are indexed only by fields. For example, to find someone in a white pages database, you need to search for information for a particular field. If you type "Mckinley" into the last name field of a white pages search, you see only people with the last name of McKinley rather than people who live on McKinley Avenue.