An arrhythmia is any abnormal heart rate or rhythm that arises from problems with the heart's electrical system. The document classifies and describes different types of arrhythmias including sinus node dysfunction, atrial arrhythmias like atrial fibrillation, and ventricular arrhythmias like ventricular tachycardia. Treatment depends on the specific arrhythmia but may include medications, catheter ablation, pacemakers, or defibrillators. Exercise is an important part of rehabilitation and can help reduce risk factors when done according to an individualized prescription.