Every country in the world has a country code Top Level Domain (ccTLD) consisting of two letters representing the nation on the internet. In some cases, there are multiple domains per country found at what are called the second and third levels ie. .uk and .co.uk. Often times, these third level domains pertain to government organizations, military, education, commerce and other geographical regions.
It is important to be certain that you are qualified to register the country code TLD that you are interested in. Some ccTLDs are unrestricted and available for anyone to register. But most require a physical presence in the country to register the domain name. Some third level ccTLDs like .gov.ng for example, have very specific requirements and many restrictions seeing as the user must satisfy requirements for both the .gov and the .ng domains. 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.