The Common Gateway Interface (CGI) is a standard or protocol for external "gateway" programs to interact with information servers such as those running the web's HTTP protocol. The CGI standard can be used in other information systems as well, but to the extent that the web is the highly dominant and most visible of all such systems, CGI may be thought of as a protocol almost exclusively found on the web.

An HTML document downloaded from the webserver and displayed on the client is static--it does not change with time, and it also cannot accept the user's input to carry out special tasks (such as accepting a query for a web search). The HTML document is a mere text file whose character and content do not change with time. A CGI program, on the other hand, is a mini-application that is executed...