A top-level domain (TLD) is the rightmost segment of a domain name, located after the last dot. In the domain name "google.com," for example, ".com" is the TLD.

TLDs serve to categorize certain elements of a website, such as its purpose, owner, or geographical area. There are several groups of TLDs, including:

1. Generic TLDs (gTLDs): These are the most common TLDs and are not restricted to specific geographic locations or purposes. Examples include ".com," ".net," ".org," ".info," and ".biz."

2. Restricted TLDs: These TLDs are restricted to specific organizations, communities, or purposes. Examples include ".gov" for government entities, ".edu" for educational institutions, and ".int" for international organizations.

3. Sponsored TLDs (sTLDs): These TLDs are sponsored by specific organizations or communities and are intended to serve a particular purpose or industry. Examples include ".museum," ".aero," and ".coop."

4. Country-Code TLDs (ccTLDs): These TLDs are based on two-letter country codes assigned by the ISO (International Organization for Standardization) to represent countries and territories. Examples include ".us" for the United States, ".uk" for the United Kingdom, and ".jp" for Japan.

5. Internationalized TLDs (IDN TLDs): These TLDs support non-ASCII characters, allowing domain names to be registered using characters from languages other than English.

You can find a comprehensive list of all TLDs in the Root Zone Database, which is maintained by ICANN (Internet Corporation for Assigned Names and Numbers) and contains authoritative information about TLDs and their associated DNS (Domain Name System) servers.

Understanding TLDs is essential when registering domain names, as it helps identify the purpose, scope, and ownership of a website's domain name. Different TLDs may be more suitable for specific purposes or industries, so it's important to choose wisely based on your website's needs and goals.