A domain is the highest subdivision of a domain name in a network address which identifies the type of entity owning the address (for example .com for commercial users and .edu for educational institutions).
A domain is the name of a website, a URL is how to find a website, and a website is what people see and interact with when they get there. In other words, when you buy a domain, you have purchased the name for your site, but you still need to build the website itself.