index.html

This file is displayed by default when someone specifies your homepage URL in their browser, without giving a certain file name.

index.cgi

This file is run when there is no index.html file, and the server needs to display a default file. If neither index.html and index.cgi exist, the server will display the contents of your directory.

*.html

These are HTML files. They are parsed by browsers to show your web site.

*.cgi

These are CGI scripts, they can be written in different languages, including C, Perl and even Java. They are executed by the server. For more information on CGI try any book on the CGI - there are many at your nearest bookstore.

*.imap

These are imagemap maps. To be used in <IMG ISMAP> tags.

*.shtml

Server-parsed HTML files, used for including server parsing in your HTML files.

*.php

Server-parsed PHP/FI files, which are in essence PHP/FI scripts embedded right into your HTML files.

*.gif,*.jpg, etc.

Images, pictures, photo, drawings, graphics, icons, etc.