TACC = Traffic Aware Cruise Control. This only handles the speed.or perhaps i'm misusing the terms .. our 2020 model 3 comes with FSD ..
so i'm calling TACC just regular old autopilot. (keeps the lane, and reacts to traffic speed).
Autosteer keeps your car centered in the lane
Autopilot is the combination of TACC and Autosteer.
FSD = Full Self Driving and adds several features (Navigate on Autopilot, Auto Lane Change, etc.)
Read all the details here.