Yourwebsite.wordpress.com versus .com
What is a domain? How is it related to my site?
A domain name is a name used to identify a particular website on the internet. Your site’s domain name is like your mailing address: it lets people know where to find you.
An example of a domain name is yourgroovydomain.com. When you register a domain (whether through WordPress.com or anywhere else) you pay for the right to use that specific domain for a specific period of time. There is only ever one yourgroovydomain.com on the internet. It would be pretty confusing if entering yourgroovydomain.com could take you to a variety of different places! Like your address.
Doesn’t make sense.
Your domain exists independently of your site, however. You can use your domain with another site, or you can cancel it, or you can let it expire, and through all of that, your site with all of its content remains unchanged.
Best of all you can register your domain with WordPress itself for a small recurring fee per year.
At WordPress.com, every site has a default web address that ends in wordpress.com (like yourgroovydomain.wordpress.com).If you register a custom domain for your site, both the original address ending in wordpress.com (yourgroovydomain.wordpress.com) and the new domain (yourgroovydomain.com) will take visitors to your site (although once they are at your site, they will only see the new domain (yourgroovydomain.com) in their browser address bar).
If your domain expires, your site with all your content will still be there at the original site address (ie, yourgroovydomain.wordpress.com).