To briefly summarize the components of a normal ECG tracings, it consist of waveform components which indicate electrical events during one heart beat. These waveforms are labeled P, Q, R, S, T and U. P wave is the first short upward movement of the ECG tracing. It indicates that the atria are contracting, pumping blood into the ventricles.
The QRS complex, normally beginning with a downward deflection, Q; a larger upwards deflection, a peak (R); and then a downwards S wave. The QRS complex represents ventricular depolarization and contraction.
The PR interval indicates the transit time for the electrical signal to travel from the sinus node to the ventricles.
T wave is normally a modest upwards waveform representing ventricular repolarization.