Sometimes an int is a bit short to store large values. Then we switch to long int, but sometimes it is still not enough. Now with C++11 we can also switch to long long int17.
This type is guaranteed to be made of at least 64 bits, while int and long int are made of at least 16 and 32 bits respectively.
Actually, I doubt I will ever use it, because when I need to store a value in at least n bits, I usually choose intn_t type from <cstdint>.
17Also available as long int long, or int long long, or signed long int long, or long int signed long, etc.