PINs are a type of password.
PINs are most often used for ATMss in which case they are 4-digit number from 0000-9999, this means that an attacker would need to guess an average of 5000 times to get the correct PIN. They are sometimes used for online systems instead of passwords severely compromising its security (see password for more details)
In 2002 two Phd students at Cambridge University, Piotr Zielinski and Mike Bond, discovered a security flaw in the PIN generation system of the IBM 3624, which was duplicated in most later hardware. This has meant most ATM's are vulnerable to an attack known as the decimalization table attack which means that someone who can access ATM hardware can guess a PIN in an average of 15 guesses.