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The Common Gateway Interface (CGI) is a standard for interfacing external applications with information servers, such as HTTP or Web servers. A plain HTML document that the web daemon retrieves is static, which means it exits in a constant state: a text file that doesn’t change.
A CGI program, on the other hand, is executed in real-time, so that it can output dynamic information.

For example, you have used a fill-out or an image map on Web, you have probably used a gateway script without even knowing it. With CGI Scripts, everything seems to happen automatically. You enter data and click a mouse button, and a moment later, a result is displayed. What happens between the click of the mouse and the display of the result is what CGI does.

steps:

.Client sends request to the server
.Server does not have the capacity to process the client request
.So, it inturn contacts some server side program like CGI
.Now CGI process the client request and sends response back to client
.Then Server sends the response back to client in the form of html page

WHAT ARE CGI SCRIPTS?
CGI scripts are external programs that run on the Web server.
CGI scripts are used to create highly interactive Web publications. The standard that defines how external programs are used on Web servers and how they interact with other applications is the Common Gateway Interface.
The three keywords that comprise the name of the standard---common, gateway and interface---describe how the standard works.
Common: By specifying a common way for scripts to be accessed, CGI enables anyone, regardless of platform, to pass information to a CGI script.
Gateway: By defining the link (gateway) between the script, the server, and other applications, CGI makes it possible for external program to accept generalized input and pass information to other applications.
Interface: By describing the external programs can be accessed by users (the interface), CGI reduces the complex process of interfacing with external programs to a few basic procedures.

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