In the context of computer science and the Internet, a domain is an area on the web made up of computers or sites that are related in some way. It consists of a set of network addresses, and top level domains are typically grouped by geographic region or a general category, such as commercial or educational. Lower level domains refer to a website's specific address.
Ex: When the Internet was first getting started, many individuals purchased domain names that they thought businesses would be interested in buying in the future.
Ex: I've recently started my own company, and now I'm looking into buying a domain for my company website. The problem is that many of the domains that I'm interested in are already taken.
Reading a URL, or web address, allows you to make some judgements about the information presented on the website. The lower level domain name is found after http:// and www. After the domain name, you will see extensions that refer to the top level domain. Extensions like .com, .edu, and .gov. indicate the type of source: commercial, educational, governmental, etc. Many two letter extensions indicate the country to which a website belongs.
Ex: The page isn't loading because you haven't typed in the URL correctly. The website is for a university in the US, so you need to follow the domain name with .edu, not .com