A top-level domain (TLD) is the last part of the a domain name. A domain name it is composed by two parts, the name and the tld, for example: nixcp.com has two parts: nixcp, the name, and .com the TLD. TLD are the letters after the final dot that is located on the domain name.
What is TLD used for?
TLD is used to identify a website, it was originally created to have a difference between domain names so visitors can identify easily the geographic location, purpose of the organization or network. TLDs are managed and designated by the ICANN (Internet Corporation for Assigned Names and Numbers).
A complete URL has a FQDN (fully qualified domain name) that includes the domain name, nixcp in this case, the TLD and the protocol that is in front, https:// in this case.
Types of TLDs & Categories
gTLD (Generic top-level domains): these are the most popular domain TLDs on the world, which include .com (commercial) .net (networks) .org (organizations) and .edu. .com .net and .org are open to be registered by anyone, however .edu are reserved for educational institudes and schools/universities only.
ccTLD (Country-code top-level domains): country TLDs have two letters only and belong specified countries, for example: BR to Brazil, US to United States, RU to Russia, CN to China, NL to Netherlands, ES to Spain. The ccTLDs are managed by IANA (Internet Assigned Numbers Authority). Countries and territories have an specific top level domain name that is based on countires ISO code.
Starting 2009, ICANN approved the use of new TLDs for specific types of markets like: .club .life .tips .soft .xxx and much more. This has generated thousands of new TLDs in the past years.
A full list of every generic top-level domain and country code top-level domain is available at IANA website.