Each computer on the Internet is identified by its IP Address (eg. 213.171.218.65). Rather than having to remember IP addresses the domain-name system (DNS) allows each IP Address to be referred to by a domain name (eg ‘www.internetmanagers.co.uk’).
Domain names are made up of a series of strings separated by dots. The right most string being the Top-Level-Domain (TLD) ie. The TLD in internetmanagers.co.uk is ‘.uk’.

It is essential to remember that you don’t actually own your domain name you are simply renting it for the duration of the term you have paid for (eg 2 years for a .co.uk). For this reason you should renew your domain name as soon as you are prompted.
If you have registered your domain name yourself you will be sent a notice usually at least 30 days before the renewal is due.
If you have registered through another company they will be responsible for renewing your domain name and may well ask you for confirmation before renewing.