A top-level domain (TLD) is the part of the domain name located to the right of the dot (" . "). The most common TLDs are .com, .net, and .org. Some others are .biz, .info, and .ws. These common TLDs all have certain guidelines, but are generally available to any registrant, anywhere in the world.
There are also restricted top-level domains (rTLDs), like .aero, .biz, .edu, .mil, .museum, .name, and .pro, that require the registrant to represent a certain type of entity, or to belong to a certain community. For example, the .name TLD is reserved for individuals, and .edu is reserved for educational entities.
Country-code TLDs (ccTLDs) represent specific geographic locations. For example: .mx represents Mexico and .eu represents the European Union. Some ccTLDs have residency restrictions. For example, .eu requires registrants to live or be located in a country belonging to the European Union. Other ccTLDs, like the ccTLD .it representing Italy, allow anyone to register them, but require a trustee service if the registrant is not located in a specified country or region. Finally, there are ccTLDs that can be registered by anyone — .co representing Colombia, for example, has no residency requirements at all.