DirectoryIndex

When you create a home page, you call it index.htm, or index.php or some simalar index.whatever .

You can change the sites index using .htaccess.

If you don't already have have a .htaccess file, create it.
Now, lets say you wanted to change the index page to "myhomepage.htm", you would add the line in your htaccess file

DirectoryIndex myhomepage.htm

If you wanted your index page to be "mobilephones.php" you would instead add the line

DirectoryIndex mobilephones.php

See the pattern?

Now what happens if you want multiple index files, say you want it to first be index.html, but if it can't find index.html it checks for mobilephones.php

DirectoryIndex index.html mobilephones.php

It checks from left to right, and the first one it finds that exists, it uses as the index page

Remember, the .htaccess file will affect the directory it is placed in, and all subdirectorys, so every subdirectory of the .htaccess will have its index set to the index you specified in the htaccess file.