Since the mid 90's vehicle manufacturers have been required to fit a passive immobiliser to their vehicles, in the most part this has been accomplished with a transponder system where the transponder is hidden inside the head of the key. These can be split into two initial categories - 'Standard Keys' & 'Remote Keys'.
Standard keys were the first iteration of manufacturer fitted transponder systems. Standard keys are a metal bladed key with (normally) a rubber or plastic head. Hidden inside the head of the key is a transponder. This transponder is used to help prevent your vehicle from being stolen.
Remote keys can come in many different shapes and sizes but the most significant difference from a standard key is the remote control function. This remote function allows you to operate your vehicles central locking remotely. Although car keys have evolved even further with the modern 'Proximity/Smart' keys the functionality is fundamentally the same.