Normal web page file names should end with “.html” or “.htm”. SSI (server-side include) files must end with “.shtml” to be parsed. CGI file names must end in “.cgi” or “.pl” (if they are written in perl).PHP file names must end in “.php” to be parsed.
If no filename is specified in a URL, the server will look for an index page, for example index.html, index.htm, index.shtml or index.php. If you have two index files, such as “index.html” and “index.php”, in the same directory, the .html file will load.
Above all, remember that filenames are case-sensitive, so index.html, Index.html, and index.HTML are three completely different file names as far as the server is concerned.