What is a Domain Name? A domain name is a virtual address that you can use for sending or receiving information such as email. It is also a location in cyberspace that you can use to hold a website.
A more technical explaination of a domain: Most computers connected to the Internet are identified by a unique number called an IP address (for instance, 126.96.36.199). IP addresses do not correspond to a geographical location nor are they easy to remember. If you type an IP address into the address bar of your browser you will be taken to the web site it relates to. IP addresses are also fixed, which means if you change web hosting companies you'll have a new IP address for your site. Domain names offer a more intuitive way to name and find a website. Each domain name replaces a string of meaningless numbers (an IP address) with a simple word or expression. That's the theory - in practice, domain names can be pretty obscure too.
The Structure of a Domain Name This site's domain name is thename.co.uk. You can check this easily by looking at the URL or address bar of your browser. ".co.uk" is the top level domain under which the domain name is registered. There are heaps of different top level domains out there, from commercial (.com) through to non-profitable (.org) and even country-specific domains such as France (.fr) which is more specific to the UK. Every domain name is registered under a top level domain of some kind. The top level domain is sometimes known as the domain extension or top domain. These are not the same thing, and are not to be confused.
"thename" is the sub-domain of the domain name thenameco.uk. This is the part of the domain name that was chosen by TheName when the domain name was registered.
The top domain and the sub-domain together make up what people call a domain name.
Always remember that when people talk about "owning" a domain name, they are really talking about the exclusive right to use that name. It is not possible to own a domain name outright, and you will have to pay a periodic renewal fee to keep this "right to use" on your domain name. Also, since you don't own the name itself, under certain circumstances a court can take your right to use a domain name away from you.
Here are a few examples of domain names that you may already be familiar with.