What's the difference between a microchip pet door and a collar-key pet door?

Microchip doors read the existing ID chip implanted in your pet (the SureFlap line is the canonical example), so there's nothing extra to wear. Collar-key doors use an RFID fob or ultrasonic transmitter on the collar (PetSafe and High Tech Pet use this approach). Microchip is more convenient for cats; collar-key systems tend to scale better to multiple pets and larger dogs.