The heart has four chambers. The two on top are called the left and right atrium. The bottom two are the left and right ventricles.
The first pacemakers were simple devices that usually acted on one chamber of the heart. The device caused a heartbeat whenever the heart paused too long. The major goal was to keep people from dying because their heart stopped beating.
A single-chamber pacemaker uses electrical pulses to stimulate one chamber of the heart, either the right atrium or, more commonly, the right ventricle.
Two-chamber pacemakers work to keep people alive and make them feel better. These devices work on the top and bottom halves of the heart. Two-chamber pacemakers deliver a pulse of electricity to the right atrium if a normal beat does not come along quickly enough. Then, after a brief pause, the device stimulates the bottom chambers. The idea is that contraction of the atria on top pushes blood down into the ventricles. This leads to more blood being ejected from the heart with each beat.
Many people feel just fine with only the lower chambers getting the electrical pulse. In some people, however, a two-chamber pacemaker can provide an "atrial kick" that improves their blood flow enough to allow them to feel better.