A top-level domain (TLD), sometimes referred to as a top-level domain name (TLDN), is the end part of a domain name.
In other words, the letters which follow the last dot of any domain name. For example, in the domain name www.example.com, the top-level domain is com or COM (domain names are not case-sensitive)

Top-level domains are currently classified into three types by the Internet Assigned Numbers Authority (IANA):

gTLD - generic Top-Level Domains:

Usually used by a particular class of organisation. Commercial organizations are .com for example. gTLD's are 3 or more letters long. Most gTLDs are available for use worldwide, but for historical reasons .mil (military) and .gov (governmental) are restricted to use by the respective U.S. and /or UK authorities. gTLDs are subclassified into sponsored top-level domains (sTLD) and unsponsored top-level domains (uTLD), e.g. .biz (business domains), .info (information domains), .org (organisation domains) and .mobi (domains for mobile device websites).

ccTLD - country code Top-Level Domains:

Used by a country or a dependent territory. It is two letters long, for example .uk for the United Kingdom. Hover over the map below for ccTLD information: