dir means that they have different subdirectory names you can choose, for instance 'sport' or 'music'. This also means that your address will get long and harder for your visitors to remember.

Domain means you can only get a free website with them if you own a domain name. Some will register a domain name for you for free, but you still have to pay to own a domain name (currently with Network Solutions $35 per year).

Subdomain is an address like this one: http://yourname.something.com