Level 0: You do everything, including accelerating, braking, steering, and anything else involved with the process of driving or operating a car.
Level 1: You still make all the driving decisions, but you get a little assistance from now-basic features like cruise control.
Level 2: You still keep your hands on the wheel and eyes on the road at all times, but the car can make some assistive changes for you, like adaptive cruise control, which can adjust speed based on the road conditions it senses.
Level 3: This level encompasses the "auto pilot" modes like you'll find in Teslas. The car can make the majority of the driving decisions under certain circumstances, but it sometimes needs a human driver to take over and make a call.
Level 4: In this case, the car drives itself—no need to hang onto a steering wheel or push pedals. However, these vehicles are limited in their scope and need consistent conditions and familiar territory that doesn't change and throw off the guidance systems.
Level 5: The car can drive itself wherever it needs to go and navigate every situation using only its systems.