Helical is curved, offset is straight. Offset is a straight fletch, but the front of the fletch does not line up with the back. And like said helical is just that helical, it rotates around the shaft to some degree.

Helical is much more aggressive than offset and will spin the arrow harder and faster. The down side is it is hard to use with a prong type launcher, especially with skinny carbon arrows. With newer arrows and a helical a drop away or shoot through rest will normally be needed. Or use feathers.

I use a slight offset myself.

Paul