There are several ways to differentiate tachycardias: based on the QRS, heart rate, and pattern. Major classifications are tachycardias with narrow vs. wide QRS. 

They are:

Narrow-QRS tachycardias: QRS < 0.12 seconds. These are primarily supraventricular (SVT):

• Sinus
• Atrial fibrillation (AF)
• Atrial flutter 
• Paroxysmal SVT
  • Atrioventricular nodal reentry tachycardia (AVNRT)
  • Atrioventricular reentry tachycardia (AVRT)
• Atrial tachycardia
• Multifocal atrial tachycardia
• Junctional tachycardia (more common in pediatric patients)

Wide-QRS tachycardias: QRS ≥ 0.12 seconds:

• Ventricular tachycardia 
  • Polymorphic VT
  • Monomorphic VT
• Ventricular fibrillation 
• Aberrant sinus tachycardia 
• Aberrant SVT 
• Preexcitation tachycardia (e.g., Wolf-Parkinson-White or WPW)