a Top Level Domain (TLD) is a domain at the highest level in the hierarchical Domain Name System of the Internet.  TLDs are generally categorized into two main groups:

gTLDs.  gTLDs are generic TLDs and the most common are the .com, .net, .org, etc. While it used to be that there were only a handful of extensions available, there are now numerous customized TLDs to choose from to have it be specific to a particular brand, industry, or type of service offering.

ccTLDs.  ccTLDs are country code TLDs and the extensions are limited to specific countries.  Countries and territories have a top-level domain name available that’s based on the country’s two-letter ISO code — for example .us.