Your domain is the unique name that that people use to find your website.
Your home and/or business has a physical address. A domain name is essentially the digital address of your business. Just like a GPS needs a street address to give you directions, a domain is what gives a web browser directions to your website.
There are two parts of a domain name. For example, my domain name misfitmedia.ca consists of the website's name (Misfitmedia) and the domain extension (.ca). The name of your website can be whatever you want it to be as long as it hasn't been purchased by someone else. As for the domain extension there are thousands to choose from. The most common are .com, .org, .net and. edu.
In order to have a domain name you will have to purchase it and re-register it annually.
Depending on who you buy your domain from, the domain extension and the sales at the time, a domain name will cost you any where from $1 - $45 per year. If you are looking to buy a domain already used by someone else, you would have to contact the domain name owner and agree on a price, this could be in the thousands of dollars depending on the domain's age and history.