The extensions on Web addresses, called top-level domains, sometimes serve as an indication of the site's origin or purpose, but in many cases, are nothing more than an aesthetic or marketing choice. Three of the most common TLDs, .com, .org and .net, are abbreviations for "commercial," "organization" and "network" respectively, but common use has moved away from these definitions, with .com becoming the most popular TLD even for sites with no commercial activity.
.Com
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The most popular top-level domain, .com, was originally designed to host commercial websites. The phrase "dot com" entered the public lexicon in the 1990s with the dot-com boom, when online start-up companies quickly rose to fame and then subsequently crashed at the end of the decade. Today, .com is treated as the de facto default TLD for Web addresses, both commercial and non-commercial, despite the availability of other options.
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.Org
The .org TLD stands for "organization," and is commonly believed to mainly host non-profit organizations — however, .org addresses don't require registrants to belong to a non-profit. Rather, .org addresses were conceived as a catch-all for sites that didn't fit into any other category available at the time in 1985: not commercial, not in networking, not a university, not government and not military. As of publication, information and management sites make up 35 percent of all .org addresses, according to the Public Interest Registry. With the popularity of .com addresses even outside the commercial sphere, however, .org never caught on as the default top-level domain in the public's mind.
.Net
In the original plans for the Internet, the .net TLD would designate sites dealing with network infrastructure. Through the mid-90s, this restriction was enforced, with .net addresses designated to "hold only the computer of network providers," such as Internet service providers and Web hosts. Today, however, .net addresses have no such restriction, and while some network companies continue to use .net addresses, the TLD is also open to anyone as an alternative to .com.
Other Top-Level Domains
These three TLDs, .com, .org and .net, are just a few of the hundreds of available top-level domains. Some TLDs have restrictions — such as .edu for universities and .jp for residents of Japan — but most are open to any registrant. Even many country-specific TLDs are open to people around the world, such as .be. This TLD was designed for use in Belgium, but is best known in the US as the extension for YouTube's URL shortener, YouTu.be.