Current course:

An "index" page is the default page when you visit a directory in your browser.

For example, if you visit http://www.google.com, the file you're actually looking at is http://www.google.com/index.html

"home" also works in most browsers, but the standard is "index".

An index file is the default regardless of it's file extension. So index.php, index.asp... etc will also work.