Cat door come in manual models that require your pet to actually push the flap open to come and go and automatic models that open using some type of magnetic or electronic "key" to open the door for your cat only.
Cat Doors make life easy for you and your cat unless they allow other animals to come into the house or let your cat out when you want to keep him in.
Automatic cat doors can be very basic: opening when you cat gets close from either direction. Or they can be very high tech. Some can use your cat's implanted microchip to identify and allow access to your pet only. Others are so sophisticated that they will tell you which cats are in and which are out, when they arrived and left, and what time you have programmed to allow passage again. Some (not many) will keep all unwanted strays out - even racoons.