In the United States, the National Highway Traffic Safety Administration (NHTSA) has proposed a formal classification system

Level 0:  The driver completely controls the vehicle at all times. This would be applicable for most “non self-driving cars” currently on the road.

Level 1:  Individual vehicle controls are automated, such as electronic stability control or automatic braking. Several passenger cars on the market feature individual automated controls.

Level 2:  At least two controls can be automated in unison, such as adaptive cruise control in combination with lane keeping. An example of a car with lane keeping is Tesla.

Level 3:  The driver can fully cede control of all safety-critical functions in certain conditions. The car senses when conditions require the driver to retake control and provides a “sufficiently comfortable transition time” for the driver to do so.

Level 4:  The vehicle performs all safety-critical functions for the entire trip, with the driver not expected to control the vehicle at any time. As this vehicle would control all functions from start to stop, including all parking functions, it could include unoccupied cars. The car Google is developing is part of this classification.