They are bell shaped and transparent. The outer margins of the bell have tentacles that carry an ecapsulated device known as a nematocyst. The nematocyst capsule contains a threadlike harpoon that when triggered entangles the prey and ingests a mild toxin. The most common jellyfish beachgoers see is the moon jelly.
This information came from the following web sites:
http://www.odc.ucla.edu.html/body_moonjelly.html
http://www.flaquarium.net/moonjelly.htm
http://www.tnaqua.org/Special/moonjelly.html