It is the rightmost component in a domain name. There are two different types of Top Level Domains (TLD): Generic and National.
A Generic TLD such as (.com, .net, .org, .biz and .info) describes the purpose of the organization or entity to which the domain name is registered. Generic TLDs do not differentiate between locations.
A National TLD (e.g. .uk, .us, .ca) specifies the country or territory of the registered domain name owner. National domains may be restricted to registration for citizens in that country. There are some domain names that do allow citizens outside of the country to register domain names (e.g. .to)