TLDs are the part of the fully qualified domain name. They are managed by Internet Corporation for Assigned Names and Numbers -ICANN.
Some well known types of TLDs :
1-) Country Code Top Level Domains
i.e .de (Germany) . fr (France) .nl (Netherlands)
2-) Generic Top Level Domians
i.e .com (commercial) .net (network) .org (organizations) .biz (business)
Among the above mentioned TLDs the most famous one is ".com" which is widely used by most of the websites. Today with the development of diffrent industries like SEO the TLD at the end seems like losing importance but ".com" still seems to be the most popular one in the following years.