Definition for different arrhythmia
Basic Cardiac Rhythms – Identification and Response (The University of Toledo, Medical Center)

This quite thorough one-page source describes:


Basic heart anatomy and physiology
The meaning of the waves in ECG
Examples of ECGs for most arrhythmias with timing, causes, symptoms and treatment


Here's Pocket Guide to Neonatal ECG Interpretation (NICU INK Book Publishers) - an one-page source about arrhythmias in infants.