OBD2 code reader vs full system scanner: what is the difference?

Common question we get from DIYers. Our team will answer below, and everyone is welcome to share their own experience.

A basic OBD2 reader costs very little, while full system scanners cost much more. What can a full system scanner do that a code reader cannot, such as ABS, airbag, transmission and body modules, live data and service resets? Which one does a DIYer actually need?