What's the difference between supraventricular thachycardia and ventricular tachycardia?

Everything. Svt arises in the atria (the upper chambers) and VT arises in the ventricles (lower chambers). Svt often occurs in otherwise healthy hearts and is annoying but only rarely serious. Vt usually arises in diseased hearts, can be serious, and sometimes is life-threatening. Vt in the setting of a heart attack or acute coronary syndrome is very dangerous.