Many Types. There are many types of arrhythmias or irregular heart beats. The most common ones are benign and not dangerous and only a nuisance at most. However if your races and does not stop or you feel poorly - lighheaded, sick to your stomach, dizzy or pass out - this could be a very serious condition. The only way to determine what kind of arrhythmia you have is to see your doctor.
Yes. Life threatening arrhythmias such as ventricular tachycardia and fibrillation can be dangerous. In general arrhythmias are not be neglected.