What is pacemaker?

The answer above is very much lengthy and useless SAN is a pacemaker i.e. it creates electrical impulses with a constant rythum and if it stops working then AVN creates electrical impulses but it doesn't works with a rhythm means that there is variation sometimes it makes heart pump at the rate 62 but on another time the rate decreases therefore it is not able to maintain the pace of heart but SAN has a tendency to maintain pace of heart that's why it is known as pacemaker and HEART of heart

Pace maker is a neuro muscular structure which produces and regulates cardiac impulses. Natural pacemaker is sino-atrial node (SA node).

The SAN is a patch of cardiac musculature tissue present in the right upper corner of the right atrium. It has the ability to generate action potentials without any external stimuli, i.e., it is autoexcitable. The SAN can generate the maximum number of action potentials, i.e., 70-75 min–1, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. 

Artificial pacemaker: is a medical device which uses electrical impulses, delivered by electrodes contracting the heart muscles, to regulate the beating of the heart.

Sometimes the SAN i.e. pacemaker gets damaged and fails to generate cardiac impluses. This defect can be corrected by installing an artificial apparatus or pacemaker in the chest of the patient. Artificial pacemaker is connected to right ventricle. The apparatus stimulates the heart electrically at regular time intervals and maintains the heart beat.