Domain name registrations for the most seen as dot extension .com, .net, .org are unrestricted and are opened to everyone. They are usually referred to as International TLDs. When the internet started some year ago, it is recommended that most of the companies should use .com, internet service providers should use .net and organizations should use .org. But today, this rules are no longer engaged and you can register under any international TLDs which you want.
Other international TLDs include .aero use most of the air-transport industry, .biz for the businesses, .coop for the cooperate .info for all uses, .museum for museums, .name for individuals use, and .pro for professions use. But they are not as popular as .com, .net and .org.
Two letter domains , such as .in repersent India , .uk for United Kingdom , .us United State and .jp (Japan) are referred as country code Top Level Domains (ccTLDs) and are corresponded to a country, territory, or other geographic location. The rules and policies for registering domain names under the ccTLDs vary significantly and some are reserved only for the citizens of the corresponding country.