I think Redline is pretty close with his pictures. From what I understand, offset and helical are pretty much the same thing only helical is more exagerated. So offset could really be called half helical. Or helical could be called offset plus.

If you look at the arrow in 2 dimensions as in a photo or lay in on the floor and look down at it, the offset fletch will start in the center of the shaft at the nock end and will be offset at about a 2 degree angle toward the tip end. You should be able to see the front and back of the vane in this 2-D view.

The helical is esentially the same thing, except it is offset at about an 11 degree angle from back to front. When you look at one of these arrows in 2-D, the back of the fletching will be in the center of the arrow, but the front will likely not be visible (or you will actually be looking at the side of the vane) since the steeper angle will take it to the side of the arrow.