What is a ccTLD?
A ccTLD is a country code top-level domain extension that is assigned to a country in the world. Each country has their own domain extension based on the ISO 3166-1 alpha-2 country codes. This means that all ccTLDs are only two characters, and all two character TLDs are ccTLDs. The first ccTLDs delegated were .US for the United States, .UK for the United Kingdom, and .IT for Italy in 1985.
Although some ccTLDs are restricted to use only by citizens of said country (an example would be Canada's .CA), many have open registration policies. Some ccTLDs have even branded themselves for alternative usage. An example would be Tuvalu's .TV domain, which is branded for online video use. Because some ccTLDs are more often used in a general way, Google and other search engines see them as "gccTLDs" instead.
Some ccTLDs also do not allow direct registrations. Instead you can only register a domain on the second-level of the ccTLD, known as ccSLD or country code second level domain. In addition, some ccTLDs offer both direct registrations and ccSLDs for registration. An example of a ccSLD would be .CO.UK and .CO.IN (both of these ccTLDs also allow direct registrations on .UK and .IN).
Please note that there are a few exceptions to the rule that all ccTLDs are based on ISO 3166-1 alpha-2 country codes. One such exception is .UK as "GB" is the UK's ISO country code.