This is the alternator. Generally, it sits beneath the power steering pump. The alternator generates power and charges the battery. On DSM alternators, the voltage regulator is built in, where as other cars may have a separate voltage regulator else where in the vehicle.
The belt for the alternator runs off the crank and water pump. If this belt breaks, your alternator will no longer charge the battery and the car will die. Your engine will also overheat quickly as nothing will be driving the water pump.
Our DSM’s eat through alternators frequently. This is because of the nearby exhaust pieces. Alternators can not handle high heat temperatures and will eventually become faulty. Heat shields can be fabricated to help deflect heat from the alternator. The alternator can also be relocated to the back of the engine in place of the AC compressor if you have removed the AC system on the vehicle.
Most turbo DSMs have a 75A alternator. Canadian models and the Galant VR-4 have a 90A alternator, which can be swapped onto our DSM. Saturn alternator swaps are also very common.