3 main parts. Generally speaking: 1) the lead that sits in the heart to detect electrical signals from heart muscle, and to conduct electrical shocks from the device (if needed); 2) the small unit in the chest wall containing the computer which interprets the incoming electrical signals from the heart (via the lead) and gives pre-programmed therapies; 3) the battery that the computer runs on (also in this unit).
2 main parts. There is the "can" which is the computer/pulse generator which monitors, analyzes, and treats the heart rhythm. The can also has a battery. The other part is the lead(s) which attach to the can and are implanted into the walls (endocardium) of the heart's inner surface.