A domain name locates an organization or other entity on the Internet.
Lets take one domain name as an example: The "com" part of the domain name is called the top-level domain name, shortly TLD. It usually reflects the purpose of the organization or entity or their location with national TLDs. There are only a limited number of such domains and currently is most popular of them .com

The "google" part of the domain name together with TLD part must be unique and is called second-level domain name (SLD). SLDs are often created based on the name of a company, product or service. 

Even more information about domains you can find in this article from Wikipedia

How you can get a domain?
Well, it is easy, you should buy it from domain registrar. There is many domain registrars and also many brokerage companies - of course better price you will get when you buy domain direct from registrar. We can recommend to you these three registrars with good services and pricing: