A Top Level Domain (TLD) is the uppermost in the hierarchy of domain names. For example, assume example.net is our domain name. The ".net" is considered the TLD and the "example.net" is considered the second level domain.
Together they form a domain name which is unique. There are two types of TLDs. The most common type is the Generic or Global TLDs which include .COM, .NET, .ORG, .MIL, .INT and .EDU. National or ccTLDs are two letter country code domains that are managed by a registry designated and controlled by each specific country. An example of this would be .co domains for Columbia. Each registry might have differing prices, residency requirements and structure.