This is the web address your site will get at the free
/you means that you will get the webaddress (URL - Uniform
Resource Locator) http://www.something.com/yourusername.
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