Top-level domains (commonly known as TLDs) are the very last part of the domain name, the rightmost section after the last period. Every part of a full domain name is separated by periods, and the periods indicate a hierarchy in the domain name system ( DNS ).
There are currently about 300 pre-defined TLDs and you can view a list of these top level domain names. Each of the 300 TLDs serve a slightly different purpose. Anyone can come up with a second level domain (SLD) for a TLD and then register it. The SLD name will usually represent a company name, a brand name, or something like that.
As an example, in www.plesk.com there are three levels of domain names. The TLD for www.plesk.com is .com, while the SLD (second-level domain) is .plesk. The third-level domain is the common www acronym which most people use to refer to a website. ICANN is the company that manages the list of TLDs.
You could divide TLDs into a couple of categories. First, you get the country-code TLDs (ccTLDs) which are two letters and which exist to serve countries around the world, of which there are over 200. This includes regions too. For example, you get a ccTLD .de for Germany (Deutschland in the local language) and .fr for France.
The remaining top-level domains are known as generic TLDs, or gTLDs. These can also be divided up into different categories – you get both sponsored gTLDs and unsponsored gTLDs. The generic gTLDs include commonly used TLDs such as .net, .com and org. Some of these TLDs can be used by anyone but for some there are restrictions. One example is .edu, which only allows registration by educational institutions located in the United States.
Finally, note that you also get what is called internationalized ccTLDs, these are domain names where the alphabet is non-Latin, for example Arabic, Chinese and Cyrillic.