What is the electrocardiogram (ECG) and what are the three waves observed in each heartbeat?

The ECG is a graphical record of the electrical activity of the heart and the conduction of its impulses.
The three waves observed in each heartbeat are the P wave, which represents the depolarization of the atria and the transmission of the impulse from the sinoatrial node to the atrial muscle fibers; the QRS complex, which represents the depolarization of the ventricles; and the T wave, which represents the ventricular repolarization.