When computers are linked together on a network they require some form of unique identification so that data can be sent to, or requested from, a specific computer. On the internet the unique form of identification used is called an IP address it consists of 4 number between 0 and 254 separated by a dot, for example the BBC web site is (at time of writing) located at 126.96.36.199. A reasonable way of thinking about IP addresses is to think of them as being a computers phone number.
A server is a computer which is permanantly connected to the internet (or other type of network) and on which are located (hosted) internet sites (domains) or other "objects" or services which are available to everyone connected to the network. A single server may host one or thousands of sites and "objects" or services.
Unfortunately as I'm sure you have all experienced it is quite difficult to memorise strings of numbers and so on the internet instead of using IP addresses directly we use the domain name system. A domain name is a string of characters that are use in the place of an IP address, for example the BBC site we mentioned before can be accessed using the IP address 188.8.131.52 or the domain name bbc.co.uk, both produce the same result but the domain name is easier to remember.
The term "domain" is used in 2 contexts. In simple everyday use (to non techies) it means a web address, or a location on the web. Thus www.planetoftunes.com is a domain.
In precise technical terms though ... .com is a top-level domain. planetoftunes.com is a domain, www.planetoftunes.com is a sub-domain of planetoftunes.com.
Originally the domain name was translated into an IP address by using a list of domain names and IP addresses stored on the computer called the hosts file, but this becomes impractical as a network grows as all computers would have to get a new version of the hosts file each time a change is made to the network. To get around the problem posed by increasing network complexity modern systems use the domain name server model, this system works by using computers that act rather like giant telephone directories. When a computer requests the IP address of a domain name what happens is the request is sent to a server that looks at the domain name and works out what other server stores the required information. The process is somewhat similar to working out someone's name from looking at their phone number, if their number is 00442073858384 you know that 0044 is the code for the UK so you get a list of UK local area codes from this list you find out that (0)20 is the code for London from this you can look up the number in a London telephone directory and find the name.
A URL (Uniform Resource Locator) is a method of finding (locating) an item (resource) on the internet. In a basic form it carries information on the domain name to locate the computer, the path the file is stored under, the file itself and information about how it is accessed.
To give an example http://www.bbc.co.uk/home/i/images/ban_home.gif has the domain name www.bbc.co.uk the path /home/i/images/ and the filename ban_home.gif and is accessed using HTTP, this mean you can locate a file (ban_home.gif) in a set of folders (in this case the folder “image” which is in the folder “i” which is in the folder “home”) on a particular server (located using the domain name). URLs can contain even more information but that is somewhat beyond the scope of this article.