Helical means the fletching is actually curved around the shaft in a helical pattern. Looking from the rear of the arrow, right helical curves to the right and left helical curves to the left. There is no difference that I know of in the two, other than right spins to the right and left spins to the left. I suspect that right and left has its roots in feathers, which are made from right wing feathers or left wing feathers. Right wing is fletched right helical and left wing is fletched left helical. Helical jigs will be specific to right or left according to the clamp. Right helical is most common and feathers are usually right wing. Vanes can be fletched either way.

Offset is just a straight fletch that is fletched slightly offset to give more arrow stabilization. Offset can be either right or left, just an adjustment made in a straight fletch jig.

If purchasing a jig, I would highly recommend a Bitzenburger. Yes, they cost a lot more than most, but yes, they are that much better than most. It will last forever and is very easy adjust and very easy to fletch with.