What is a TLD ?

A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain.

The top-level domain names are installed in the root zone of the name space.
For all domains in lower levels, it is the last part of the domain name, that is, the last non empty label of a fully qualified domain name.
For example, in the domain name www.example.com, the top-level domain is com.

Responsibility for management of most top-level domains is delegated to specific organizations by the ICANN, an Internet multi-stakeholder community, which operates the Internet Assigned Numbers Authority (IANA), and is in charge of maintaining the DNS root zone.

What is a ccTLD, sTLD & gTLD ?

ICANN identifies the following categories of top level domains:

Country Stop Domains (ccTLDs) Each ccTLD identifies a specific country and is two letters long. Sweden, for example, has .SE as its top-level domain. Here you can register European domain names, African domain names, Asian domain names, American domain names, South American domain names, Top-level domains in the Middle East. Infrastructure Top Domain - There is only one TLD in this group, ARPA (Address and Routing Parameter Area). The Internet Assigned Numbers Authority (IANA) handles this TLD for the IETF.

Sponsored Top Level Domains (sTLDs) These are administered by private organizations. Examples of sponsored top level domains are .AERO, .COOP and .MUSEUM.

Generic Top Level Domains (gTLDs) These are the most common and well known top level domains. Examples are .COM for "commercial" and .ORG for "organizations". Most gTLDs are open to registration by anyone, but there is also a subgroup that is more strictly controlled.

All TLD's IANA List
IANA List of TLDs on the DNS Root Zone