Domain extensions, also known as top-level domains or TLDs, are the suffixes or the last part of a domain name - the letters that come after the dot to the right of any domain name - for example .org 
It is the part of a network address which identifies it as belonging to a particular domain.
When the internet first began there were relatively few choices of extensions. Most businesses used .com or .org as an alternative. There many are domain extensions for all types of industries now.
There are several different types or categories of top-level domains (TLD's) within the Domain Name System (DNS) maintained by the Internet Assigned Numbers Authority.