ccTLD are a type of Top Level Domain; which are referred to as TLD. The first Top Level Domain extensions were: com, net, org, mil and gov. These have subsequently gone on to be called Generic Top Level Domains; it would be fair to say that gTLD's are the "core" domain names, and, as such, are viewed as being the most important.
Domain names are part of the Domain Name System (DNS): which is a hierarchical naming system for namespace on the Internet. Domain names are used as easy to understand "nicknames" which point to a resource at an IP address; the domain name system converts domain names into an IP address.
ccTLD stand for 'country code Top Level Domain', and as you would expect, are used for individuals and companies who target content and services (on the Internet) for a specific country. The Internet evolved from ARPANET - a US based computer network - therefore, the Internet evolved in the United States. The vast majority of organisations and agencies who administer how the Internet functions are based in the US. ICANN is one such organisation: ICANN maintains the domain name system. It is also why the United States tend's to use gTLD's rather than ccTLD's.
The first ccTLD created, were in created in 1985, and were for the following countries:
In 1986, eight more ccTLD's were created, such as fr (France) and de (Germany).
At present (2014) few recognised countries do not have a ccTLD; some of the most popular ccTLD's are listed below:
ccTLD's typically comprise a second level domain, for example, in the United Kingdom: