Large networks may have thousands of systems connected to them. A domain name addressing scheme is used to specify system addresses in a way that humans can understand.
The top level domain for the UK is uk. This domain has several sub-domains; ac (for academic) and co (for commercial).
The ac domain has many sub-domains, based on the name of the organisations within it. The sub-domain for the University of Hull for example is hull.
If the organisation has a large number of systems, then the organisational domain might also contain sub-domains, each of which contains a number of systems. This is often done on a departmental or service basis.
The mail address for any system can be given as a series of domains, each of which is separated by a dot (full stop).
For example, the full mail address to a user on a particular system might look like this:
john@ubik.ucs.ed.ac.uk
Reading from left to right, this is the mail address for the user known as john on the system called ubik. This system is in the organisational sub-domain ucs, (University Computing Service) which is in the organisational domain ed (Edinburgh).
This organisation is located in the ac (academic domain) for the national domain uk (United Kingdom).