Every nation in the world has a country code Top Level Domain (ccTLD) consisting of two letters representing the country on the internet. In some cases, there are several second and even third level domains that pertain to government organizations, military, education and commerce within the country. In other cases, third level ccTLDs based on geographical regions are also available, depending on the country.
It is important to be certain that you are qualified to register the country code TLD that you are interested in. For some, the domain name may be unrestricted for anyone to register. But for most, the ccTLD will require a presence in the country to register the domain name. Second level domains like .gov and .mil are even more restricted within a ccTLD that have very specific requirements and many restrictions.
Many of these ccTLDs are also available as Internationalized Domain Names, which allow the ccTLD to be created in non-Latin characters. This allows for the ccTLD to be registered using characters that are native to the alphabet of the country in question. Once you have located the country and ccTLD you are searching for, you can click on it to see all the associated second and third level domains that are available.