A Deadlock is a mechanical device fitted to a vehicle door. It operates by throwing a bolt into a receiver fitted to the opposing body section when operated by an external key. These locks must be operated manually which prevent you from being able to lock keys within the vehicle but it also makes it easy to forget to secure vehicle.
A slam lock is a device that automatically locks the vehicle door immediately it is shut, without requiring any operation by the driver. As these lock automatically when you close the door there is no chance you will forget to secure vehicle however it increases the chance of locking keys within the vehicle.