What is a TLD?
A TLD is a Top-Level Domain. The internet addressing system, called the Domain Name System, enables every page of every web site to be reached from anywhere in the world. This is achieved by creating an address for every domain on the internet.
If you consider your home address, for example, the TLD is like the country used in your home address and enables the correct registry to be found quickly to identify where to send traffic looking for your web site.
A TLD is the last part of a fully-qualified domain address, such as .com, .eu, or .uk. These are all second-level TLDs, because they come immediately after the domain address (e.g., redcentaur.com is a domain name + second-level TLD). Some registries also support third-level TLDs, such as .co.uk, .me.uk, etc., which add the TLD a level lower than in the second-level TLDs.
Responsibility for management of most TLDs is delegated to specific organisations by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA). This is what makes the internet work, ICANN sets the rules required for assigning names and numbers while the individual registries set the specific requirements for their particular TLDs. All domains using the internet are required to be registered so that they can be found from anywhere in the world and properly addressed according to the standards set by ICANN.