A Top Level Domain is a term used to describe the last part of an Internet domain name. To put it simply, this is the last part that appears after the dot. For instance, when it comes to yahoo.com, the top level domain is com. Although they are short and people usually don’t pay a lot of attention to them, the truth is that they are managed in a complex way. Their management is in the hands of the ICANN (Internet Corporation of Assigned Name and Numbers) which is in charge of the IANA (Internet Assigned Numbers Authority) that keeps the DNS root zone operational all the time.
The basic role of IANA is to separate top level domain names. Country-code top level domains or ccTLD are the domain names that are used by countries. On the other hand, sponsored top level domains or sTLD are sponsored domains by organizations or agencies. There are also gTLD or Generic top level domains which can be used by any individual or organization. Some experienced people in this field, are pointing out that there are generic restricted top level domains too.
In its essence, a top level domain is the non-restricted domain extensions that anyone can register. What is important to know is that ICANN is reviewing TLDs all the time and every now and then they are adding some of these top level domains to the gTLD list. Not while ago, ICANN has recommended that the .biz, .aero, .name, .museum, .info, .coop and .pro to be added to this list. The fact is that many of these top level domains have thousands of attractive domain names that are still available. In addition, there are many initiatives to add more top level domain names to the list. Many of these suggestions are related to the adult entertainment and casino industry like .casino or .adult or .xxx. We should also mention that ICANN has some reserved TLDs that are not available to the public like .test, .localhost, .invalid etc.
In case you want to register a brand new domain extension you have to contact ICANN and ask for a TLD. But, even if they process your application, it is very unlikely that the new domain extension will be approved.
The number of Top level domains (TLD) is growing which is quite natural because the number of websites is growing too.