CGI stands for Common Gateway Interface. CGI is one method by which a web server can obtain data from (or send data to) databases, documents, and other programs, and present that data to viewers via the web.

More simply, a CGI script is a program intended to be run on the web. A CGI program can be written in any programming language, but Perl is one of the most popular. On HostGator servers you will get a cgi-bin directory that your scripts should normally be uploaded to, however your CGI scripts will work both inside and outside of this directory.