A top-level domain (TLD) is the part of a domain that comes after the dot, for example, com, org or net.
Generally, you can divide TLDs into two types:
- Generic top-level domains (gTLD) - Roughly all domains that are not associated with a country. The most known are com, org and net. All gTLDs fall under ICANN's policies.
- Country-code top-level domains (ccTLD) - All domains that are identified with a country or geographical location, for example, de, nl or dk. A ccTLD always consists out of two letters.
Over the last years, a lot of new gTLDs have become available, for example, .agency, .email or .house. At one.com you can choose from more than 250 different TLDs.
Check out the wikipedia article about top-level domains for more information.
When you choose your domain, you need to find a unique combination of the bit before the dot (second-level domain) and after the dot (top-level domain).