P wave shows the process of atrial depolarization.
U wave is caused by repolarization of the interventricular septum with lower amplitude. They normally go in the same direction along with the T wave. Patient may suspect hypokalemia in case these waves are too prominent.
QRS shows ventricular performance of the left and right ventricles.
The PR interval can be measured from the beginning of the P wave to the QRS complex. This interval tells the total time the electric impulse takes to enter ventricles after travelling from sinus node through the AV node.
PR segments connects QRS complex with P wave. However, PR interval is regarded as more important and relevant in clinical terms.
QT interval can be measured right from the beginning of the QRS complex to the end of the T wave. A prolonged interval may be life threatening.
ST segment connects the T wave with QRS complex. This segment tells the time period when the ventricles are depolarized.
T wave shows the process of rapid repolarization.