In general what most people wear under their wetsuit is a swimsuit for the girls and a small speedo or boxer short for the guys or they wear nothing underneath (just naked).
A zipless or zipperless or zip free wetsuit are all the same name for the same wetsuit. But what is it and how does the wetsuit work? Zipless or zip free wetsuits are the same as a normal wetsuit only there are no zippers involved when putting the wetsuit on.
A back zip wetsuit is a rubber bodysuit that keeps you warm when you're in the water for an extended amount of time. It is called a back zip wetsuit because of the vertical zipper at the back.
There are three types of wetsuits: chest/front zips, rear zips, and zip-free. The front zip is much like any other wetsuit, however, the zipper is located at the front of the wetsuit (on the chest). The way you put on a chest zip wetsuit differs significantly from other models.