A domain is a human-readable name assigned to an IP address to make accessing websites much easier. Domains are combined with a TLD to produce what is known as a domain name. Domains themselves can be anything you choose (bobsflowers, ratloverforum, etc), and then choose a TLD to go with it. Common TLDs include:
Your domain, combined with a TLD, will procude the final domain name (bobsflowers.com).
Domain names must be unique, and can only be registered to one person at a time.