Jason pretty much nailed it, I'll add to the dancing a little, if the offensive player engages the defender with the hands inside and the defender does nothing to get away and goes with the offensive player this is also dancing, because the defender is going right along as if being led in a dance.
This is probably the most common area where the offensive player may have hold of jersey or pads inside the frame of the body, but there is no restriction since the defender displays no effort to get away.
One key thing to watch for is the feet or gait of the defender, if there is a hold they will typically "lose their feet". So if the defender is running along with the play going east and suddenly his motion slows or stops dramatically the he is being restricted from where he wants to go.
Also like Jason mentioned, POA. If the play is a sweep to the left, if the right tackle has a little jersey tug on the backside of the play that shouldn't be called.