Country tld are generally registered with that country's agent, which is usually a contracted ICANN registrar, versus a .com/.net/.org that can be registered using most ICANN accredited registrars. In some cases the registration for a country tld may be limited to it's citizens or business's. This allows company's to register domains that may not be available as a .com, but are available as a .com.ar and which may be more appropriate since it targets a specific region. Google, for example, has google.com for the world, but also has google.com.ar with a default language of spanish.