I’ve seen domain names ending with two-letter combinations, like .uk. What are the rules for registering in these domains?

Two letter domains, such as .uk, .ae (for example), are called country code top level domains (ccTLDs) and correspond to a country, territory, or other geographic location. The rules and policies for registering domain names in the ccTLDs vary significantly and some are reserved for use by citizens of the corresponding country. You should check with the registrar offering ccTLD registration services regarding the specific terms and conditions for registration. Some ICANN-accredited registrars provide registration services in the ccTLDs in addition to registering names in .aero, .biz, .com, .info, .net, .org, however, ICANN does not specifically accredit registrars to provide ccTLD registration services.