ICANN and gTLD
ICANN (the Internet Corporation for Assigned Names and Numbers) is a non-profit making international corporation based in California. In November 1998, it took over the role from the US Government on administering the generic top-level domains (gTLD) of .com, .net and .org and other 4 gTLDs.
There are no restrictions on who may register .com, .net and .org. The other four are restricted. Only organisations meeting certain criteria may register names bearing those gTLDs. They are .int (international organisations); .edu (colleges and universities); .gov(agencies of the US government); and .mil, (military of the USA).
Apart from those few gTLDs, there are at present 243 ccTLDs. Each of them bears a country code represented by 2 letters, such as .cn (China), .hk (Hong Kong), .uk (United Kingdom), .au (Australia), and so on.