This again is where a number of newer webmasters become stumped. 

They upload all of their files and directories, and then want to access them with their browser, but forgetting to create their welcoming page as index.html,  this is what happens: 

They access their site as:

http://www.yourdomain.com/ 

and what they see is their entire file directory structure! 

It looks just like exploring the C drive on your computer! 

You don't want visitors seeing that, do you?

When you access your site the web server looks for the "index.html" file as the (default file) to be sent to visitors, and thus this is why

http://www.mydomain.com 

by itself will automatically display the home or welcoming page. 

It's because the server automatically looks for index.html whenever a domain or directory is called without a filename appended to it such as this, http://www.mydomain.com/file.html

If it can't find index.html, it will simply list your entire web directory to everyone that access's it, which is a MAJOR security risk!

ALWAYS, use an "index.html" file in any directory you create, including your "public_html" web directory. 

In general, it's always a good idea to use "index.html" as your main page in "all sub-directories" of your account.