The jelly like skin on the jellyfish will open up like an umbrella. This allows water to move in. The jellyfish will close the skin and push the water out, causing the jellyfish to move up. You can find more information here: http://library.thinkquest.org/CR0215242/jellyfish.htm
Jellyfish, or Medusa (plural: medusae), a water-dwelling animal. It is not a fish but a cnidarian, a spineless, soft-bodied animal. The jellyfish is one of two basic body forms that the animal has during its life cycle; the other is called a polyp
Jellyfish look as if they are just floating but they actually have a way of moving that consists of being in water and floating on the current. Jellyfish are mostly water anyway, so floating with the ocean current is easy for them.