The Queen Jellyfish is, as the name implies, the "queen" of all the jellyfish. The Queen appears at the end of the episode: Jellyfishing and stings an already injured, Squidward. Also the jellyfish is only mentioned in the episode: I'm Your Biggest Fanatic, as Kevin C. Cucumber uses a machine to imitate the queen and scare SpongeBob.
The queen jellyfish just looks like a regular jellyfish, except much, much bigger. Also its sting is more powerful.