To put it simply, domain is the address of your website. The one which user enters to the browser’s address row when he/she wants to access your site.
Domains are alphanumeric names that are used to refer to machines connected to a network in a more memorable way than numeric IP addresses. The domain can be 3 to 60 characters long and can contain the letters a-z as well as the – character. A domain consists of a prefix, a body, and a suffix. The prefix is most commonly www.