A rebuilt motor (aka remanufactured motor) typically consists of new gaskets, piston rings, bearings, camshaft, lifters, connection rod bearings, valve train, timing belt, gears and rod.
Usually the crankshaft, connecting rods, valves and valve seats are all remachined. Also the remanufactured motor will have all new fuel filter, thermostat, anti-freeze, oil filter, oil change, motor mounts, spark plugs, air filter, hoses and belts and water pump.

When buying a car with a rebuilt motor ask the seller for documentation on what was rebuilt and who rebuilt it.