The Common Gateway Interface, or CGI, permits interactivity between a client and a host operating system through the World Wide Web via the Hyper Text Transfer Protocol (HTTP). It's a standard for external gateway programs to interface with information servers, such as HTTP or Web servers. A plain HTML document that the Web server delivers is static, which means it doesn't change. A CGI program, on the other hand, is executed in real-time, so that it can output dynamic information - perhaps a weather reading, or the latest results from a database query. CGI allows someone visiting your Web site to run a program on your machine that performs a specified task.