Generic Top-Level Domains
The most used high-rank domains are .com, .org and .net. Although we might not completely understand the difference nowadays, they all had a specific purpose earlier.
.com was used for commercial websites like stores or restaurants.
.net was used for domains related to networks, such as some specific email servers.
.org was used for non-profit organizations. Perhaps this is the only domain that is still used for that specific purpose.
Nowadays, these domains have open registration, which means that anyone can register using a .com, .net or .org domain. Although .org is still widely used for organizations, .net is a domain that is rarely seen for the purpose of network related domains as it was originally intended.
As the Internet kept growing, more domains were added, such as .biz, .info or .gov. However, not many websites use these domains in comparison to the top level domains (.com, .net, etc.). There is more recognition for websites using .com domains. According to Google, around 50% of websites use the .com domain. (Source)
Open vs. Closed TLDs
Whereas the top-level domains we have mentioned before are open to anyone registering a specific domain, there are also some top-level domains, or TLD’s that are closed. Domains like .museum, .travel or .aero are domains only available to those who go through a verification process to ensure they are in fact a museum, airline or tourism company.
Country-Specific Top-Level Domains
There are also country-specific domains such as .uk for the United Kingdom, .ca for Canada or .mx for Mexico. Some of these are only open to people living in the country or business with origin in the country chosen. However, there are some that allow open registration to citizens or business from other countries if they wish to promote in the chosen country.
An example of a country-specific domain that allows open registration is .ly, which is commonly used in bit.ly and similar shorter URL’s. .ly is the domain used specifically for Libya, but it allows open registration under some restrictions in regards to the content available in the website.
The United States has it’s own domain called “.us”, as well as others, such as .gov, used for governmental websites, .mil, specifically for military use and .edu, specifically for educational institutions.
Future Top-Level Domains
In 2012, ICANN allowed companies to apply for a specific top-level domain. Google applied for domains like .google, .youtube or .docs. Companies like McDonalds and Apple also applied for specific TLD’s using the company name. Subsequently, other companies applied for domain names that are not brand-specific, such as .pizza or .beer. And although some of these domains are not in use yet, we could expect to be seeing new top-level domains just around the corner.