Level zero: The human has complete control over their vehicle including the steering, breaking, and throttle. Essentially the way driving is done today.

Level one:  Small elements of driving such as braking can be done automatically by the car.

Level two:  At least two functions are automated such as lane centering and cruise control. In this phase the driver can have their hands off the steering wheel and foot off the pedal, however they still must always be ready to take control.

Level three:  Although the driver is still present at this level, they aren’t required to monitor the environments around them as much as they would in previous levels.

Level four:  According to the Department of Transportation (DOT), vehicles at this level are “designed to perform all safety-critical driving functions and monitor roadway conditions for the entire trip.”

Level five: The vehicle is fully autonomous and has no steering wheel or general controls for a human to use.

Here’s a chart that summarizes and expands on the above. The full details of these standards can be found in SAE Standard J3016.