Steven Haddock of the Monterey Bay Aquarium Research Institute (let's all pause a moment to reflect on kismet of that surname/job combination) made this video about the wide world of creatures that we call "jellyfish." It's a great summary of the extreme diversity encompassed under one, catch-all name, and does a really nice job of explaining relationships between different species and families of jelly-like creatures.
And let's not discount the stop-and-gawk value. Check out 1:27 for a Hydromedusae that looks strikingly like Darth Vader's helmet. You'll also meet shell-less snails, jelly worms of the sort you don't find at the candy store, and even colony-dwelling creatures thousands of individuals strong.