Well
Beginner
A zone of the silicon given a particular mix of added atoms, so one kind of switch can be built in it.
Novice
A doped region a few micrometers deep: NMOS transistors sit in a p-type well, PMOS in an n-type well. Both kinds on one wafer is what makes CMOS possible.
Expert
Formed by masked implants, today high-energy retrograde implants whose concentration peaks below the surface, which lowers well resistance and avoids a long high-temperature drive-in.