A domain or "domain name" is the address used to access your website on the internet and all its related services. It allows people to access your site via a memorable word or string of words without having to input complicated computer commands. As such whoever controls your domain also controls what can be set up on the domain such as a website, email services, ftp servers etc. 

Examples of domain names include: "itseeze.co.uk" "bbc.co.uk" and "facebook.com"