top and bottom end

Top end:
Cylinder, head, piston, rings, power valve, anything contained above the cases.

Bottom end:

Clutch, flywheel, crankshaft, rod, transmission, anything contained in the cases.

Usually if someone says they are doing the "top end" they mean replacing the rings, and/or piston.

If they are doing the "bottom end" they are usually refering to replacing the main bearings and or rod and its lower bearing.

Hope this helps.

and crank seals. I mean, might'as well.