A domain is a human-readable name assigned to an IP address to make accessing websites much easier to remember, and relevant to the business. Domains themselves can be anything you choose, for example ours is evolutionwebdesign and then you choose a TLD to go with it ( the end bit). We have chose .co.uk since we are based in the UK other common TLDs include:
Your domain, combined with a TLD, will produce your final domain name in our case its evolutionwebdesign.co.uk.
Domain names must be unique, and can only be registered to one person at a time. Its best to try and keep them as close to your interest / business name as possible.