ECG or Electrocardiogram - It is the the graphical representation of the electrical activity of the heart during a cardiac cycle. In this the electrical activity of the heart is recorded over a period of time using electrodes placed on the skin. 

The peak in the ECG is identified with a
letter from P to T that represents the specific
electrical activity of the heart.

The P-wave represents the electrical excitation (or depolarisation) of the atria. The QRS complex represents the depolarisation of the ventricles. The T-wave represents the repolarisation. The end of the T-wave marks the end of systole.