Without domain names, you would have to memorize the Internet Protocol, or IP, address of every website you frequently visited. Of course, there is a certain hierarchy to domain names, just as there is for IP addresses, which helps better organize the Internet.

Top Level Domains

Top level domains (or TLDs) are the final part of a domain you write. This part of the domain is the part that usually ends with either ".com," ".net," ".edu," ".gov" or other names, in the case of General TLDs (or gTLDs).

Other TLDs

There are other kinds of TLDs called ccTLDs (country code TLDs). These are the websites you see with ".us," ".eu" or some other code representing a country. They were created for websites specifically targeting an audience from a specific nation.

Second Level and Third Level

The second level domain is often called the "primary domain"--for example, in the domain name "name.com," the word "name" is the primary domain name. The third level domain would be the "subdomain," which indicates exactly what it implies: a domain under the primary domain ("img.name.com," with "img" the subdomain).

Benefits

Domain names provide a sense of simple order out of all the chaos of IP numbers on the Internet. It is much simpler to remember "subdomain.domain.com" than "129.255.3.13".

How Domains Work

Based on the domain name entered on a browser, the computer from which the request was sent sends a request for the IP address of the domain name. Ultimately, you are still connecting to an IP, but you typed in a domain name to get to that specific server.

Source:

ICANN: Top Level Domains