What is a natural pacemaker ? What is artificial pacemaker?

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.