Tune up: Cleaning, lubing, derailleur tuning and adjustment, brake tuning and adjustment, sometimes new cables, tape and brake pads [pay extra].
Overhaul: Repack/replace bearings, disassemble, clean, lube and reassemble components [specifically RD cage and pulley wheels], new chain and cassette... and all the things in a tune-up.
Tune ups are rarely worth the fee if you do your own work. Overhauls are nice if you don't have the time or inclination to strip and rebuild your bike.