There are basically two types of handpieces, air-driven (high-speed) and electric (low speed).
High-speed drilling is one of the main reasons teeth become sensitive to hot and cold, and can cause the death of teeth. Teeth, sensitive after high-speed drilling, can have pain that lasts from days to months.
Electric (low-speed) handpieces are much stronger and slower than air-driven due to the fact that a motor is spinning the bur as opposed to air. This allows the doctor to remove decay at a faster more controlled pace. This will cut down on the time it takes to complete a procedure and the amount of trauma caused to the tooth.