This post is using ACLS 2010 (AHA) Guidelines recommendation:-
To make things simple, arrhythmia that originating from atrial need lesser power (joules) of cardioversion as to compared with those from the ventricular in origin.
How we differentiate this? simple. Look at the QRS complexes. If the QRS complexes are narrow, then it is atrial in origin, but if the QRS complexes are wide, (more then 0.12sec) then it is mostly a ventricular in origin. Got that? :)