Pacemakers
A pacemaker is a device designed to regulate the beating of the heart. The purpose of the pacemaker is to stimulate the heart when it is not beating fast enough or if there are blocks in the heart's electrical conduction system preventing the propagation of electrical impulses from the native pacemaker to the lower chambers of the heart.
Defibrillators
Defibrillators (also called ICDs [Implantable Cardioverter-Defibrillators]) include all of the functions of a pacemaker, but are also able to shock the heart to get it back into rhythm.
CRT-Ds
A CRT-D (Implantable Cardiac Resynchronization Therapy Defibrillator) system is an ICD with an additional lead that helps both sides of the heart beat in sync and thus pump more efficiently.