CGI is the Common Gateway Interface used by web HTTP servers to interact with server-side programs and utilities. The most common use for this is for handling forms used on web HTML pages.
The process typically goes as follows:
CGI defines how the information is passed between the server and utility. While CGI defines how this is done, various HTTP server implementations have handled this differently - sometimes due to operating system limitations.
In some implementations, the information is passed via command line arguments, others use standard input/output (stdio) conventions, and still others use system environment variables. Many support a combination of all three methods.
The utilities that support CGI may be compiled programs, interpretive (PERL), shell scripts or even DOS batch files. As long as the CGI implementation supports standard IO, anything that read/writes via stdio can be a back-end to HTML forms.