There are a range of cat flaps available, the main difference usually being how they are activated.
Standard cat flaps open for any cat and, while the cheapest option, they will allow neighbourhood cats in to have a look around! This can be very stressful for your cat.
Magnet cat flaps will open if the cat trying to enter has a magnet on their collar. This does restrict entry only to cats that have a magnet, but isn’t exclusive to your cat.
Infra-red cat flaps are similar but are programmed to a specific coloured infra-red tag.
Microchip cat flaps are the best available as will only let in cats whose microchip numbers have been programmed in. This means no other cats can enter, no matter what is on their collar! It also means your cat will never be able to ‘lose their key’, as the microchip is always with them.