In the Domain Name System (DNS) hierarchy, it is the highest level under the root. In a domain name, it is that portion that appears furthest to the right; for example, the "com" in www.netlingo.com.
Two letter domains, such as .uk, .de and .jp (for example), are called "country code top-level domains" (ccTLDs) and correspond to a country, territory, or other geographic location. The rules and policies for registering domain names in the ccTLDs vary significantly and some are reserved for use by citizens of the corresponding country. You should check with the registrar offering ccTLD registration services regarding the specific terms and conditions for registration.
Be sure to also read through these definitions: .com, .edu, .gov, .net, .org, country codes
The official list of all top-level domains is maintained by the Internet Assigned Numbers Authority (IANA).