The Common Gateway Interface (CGI) provides a mechanism for a program on the server to interact with the client’s browser. You can use any language to write CGI programs, and CGI programs may be interpreted (PERL scripts, for instance) or compiled (C or C++).

One popular use of CGI is in hit counters-programs that modify the page to show how many times that page has been visited. Another popular use of CGI is in form handling, where a program on the server reads the data from the user input fields and does some work based on that data.