Internet addresses follow particular conventions when it comes to their domain names. The last part of a website's address -- the .com in Synonym.com, for example -- is the top-level domain, or the broadest category of Internet and Web addresses. This system, dating back to Internet RFC 940 in 1984, grew out of earlier conventions used for ARPANET. Some of the earliest top-level domains were .com, .gov, .mil and edu.
The earliest top-level domains used on ARPANET, the predecessor of the Internet, belonged to the U.S. military and government. These top-level domains were different because in order to get one (or get an email account tied to one), you needed to prove that you worked for the military or the government. As ARPANET grew and became a channel of communications for researchers, universities were granted the .edu domain. Originally, access to the .edu domain was limited to professors and faculty, but it didn't take long before access to email accounts filtered down to college students.
While .gov and .mil had organizations dedicated to ensuring that only properly accredited organizations got the rights to set up Web servers using them, the original specification also opened the door for commercial use of the ARPANET infrastructure. Originally, to get a .com address, you needed to prove that you were a commercial entity. Similarly, .org was limited to non-profit or community-based organization, while .net was created as a "generic" domain name that anyone could access. Due to a lack of a central authority to enforce these requirements, they quickly became generic top-level domains and anyone could get them. Indeed, for most of the 1990s, there was money to be made in registering .com, .net and .org addresses for trademarked or generally useful terms. The business model was to register the domain name, put up a website that painted the trademark holder in a mildly unfavorable light, wait for them to contact you and sell them the domain name.
It's important to note that regardless of the top-level domain, there aren't any restrictions on what top-level domains you can see or visit with your Web browser. For example, you don't need to be logged into a military computer to browse www.navy.mil or be logged into a .edu account to view www.harvard.edu. Where the restrictions lie is on who's permitted to register a domain name in those categories.
In 1998, after authority for creating new top-level domains transferred to the Internet Corporation for Assigned Names and Numbers, several new top-level domains were opened up. Some, like .info and .biz were intended to expand the range of names for businesses and non-profit organizations, and were generally available to anyone without restriction, while .aero domain names are restricted to air transport industry. Since 1998, other domain names have been approved, including .cat , which is dedicated to content honoring Catalan culture, and the controversial .xxx domain name for adult websites.