P-wave: a small low-voltage deflection caused by the depolarisation of the atria prior to atrial contraction.
QRS complex: the largest-amplitude portion of the ECG, caused by currents generated when the ventricles depolarise prior to their contraction.
T-wave: ventricular repolarisation.
P-Q interval: the time interval between the beginning of the P wave and the beginning of the QRS complex.
Q-T interval: characterises ventricular repolarisation.