OK, here's my attempt to describe the difference.
Helical fletching actually curves the feather around the shaft helping the arrow to spin. Some jig settings allow for more helical than others.
Straight fletching places the feather straight down the shaft (parallel to the sides) and straight up and down. No real adjustment except for the number of fletchings.
Offset fletching places the feather straight up and down but somewhat accross the shaft (not parallel to the sides of the shaft). This also helps the shaft to spin. Most jigs allow you to adjust the amount of offset.
Because the feathers are placed accross the "grain" of the shaft, size sometimes dictates the amount of helical or offset you can do with the fletchings.
Hope that helps. Maybe others can describe this a bit better.