Domain Names are the cornerstones of the internet. Without access to a domain name you cannot have a working email address or a website. An explanation of the way these items hang together can be seen below:

Let's assume you intended to buy the domain cars4hire.co.uk

Domain Name - cars4hire.co.uk

This is the name itself, and buying the domain name is only the first step.

Email Address - sales@cars4hire.co.uk

This would be an email address using your domain and the sales@ at the beginning denotes email.

Website - www.cars4hire.co.uk

This is the website address and people could view the site at this location.
The www at the beginning denotes a website.

So you can see that in all three cases the central theme is the domain name itself, cars4hire.co.uk.

You can buy domains, sell them, transfer them or use them in the ways shown above. Every domain is administered by a registry, and an ISP like Skymarket usually deals with them on your behalf.

You can find links that will enable you to do all of the above, here