Level 0 - The human driver does all the driving. Level 1 – An advanced driver assistance system (ADAS) on the vehicle can sometimes assist the human driver with either steering or braking/accelerating, but not both simultaneously. Level 2 - ADAS on the vehicle can itself actually control both steering and braking/accelerating simultaneously under some circumstances. The human driver must continue to pay full attention (“monitor the driving environment”) at all times and perform the rest of the driving task. Level 3 - An automated driving system (ADS) on the vehicle can itself perform all aspects of the driving task under some circumstances. In those circumstances, the human driver must be ready to take back control at any time when the ADS requests the human driver to do so. In all other circumstances, the human driver performs the driving task. Level 4 - ADS on the vehicle can itself perform all driving tasks and monitor the driving environment – essentially, do all the driving – in certain circumstances. The human need not pay attention in those circumstances. Level 5 - ADS on the vehicle can do all the driving in all circumstances. The human occupants are just passengers and need never be involved in driving.
https://www.nhtsa.gov/technology-innovation/automated-vehicles-safety#topic-road-self-driving
Figure 1 SAE Levels of Automation (Source Poorsartep) https://static.tti.tamu.edu/tti.tamu.edu/documents/PRC-2017-5.pdf