What are the key differences between premature atrial contractions (PAC) and premature ventricular contractions (PVC) as seen on an electrocardiogram (ECG)?

Premature atrial contractions (PAC) are early heartbeats originating in the atria, while premature ventricular contractions (PVC) are early heartbeats originating in the ventricles. On an ECG, PACs typically show a P wave that looks different from the normal sinus rhythm, while PVCs show a wide and bizarre QRS complex.