They are very similar in my view: they automate part of driving to reduce load on the driver. Traditional cruise control just maintains velocity. My Subaru's adaptive cruise control maintains velocity and controls distance relative to the car ahead. Tesla also keeps your car centered in lane, but is otherwise just adaptive cruise control. There's some more features in the "self-driving" cars, like auto-parking and overtaking - I don't have much experience with those, I've only used non-self-driving Tesla autopilot. For the normal Tesla autopilot, though, "adaptive cruise control plus lane-centering" seems to accurately describe it.