Jellyfish are not really fish. They belong to invertebrate animals. It means that unlike fish generally or people, they have no backbones. In fact, they have no bones at all.
Jellyfish have stomachs and mouth, but no heads. They have nervous system for sensing the world around them, but they have no brains. They are made almost entirely of water, which is why you can look through them.
Some jellyfish can glow in darkness by making their own light. The light is made by a chemical reaction inside the jellyfish. Scientists believe jellyfish glow for several reasons. They may glow to scare away predators or to attract animals they want to eat, for instance.
Most jellyfish live in salt water, apart from a few types that live in fresh water. Jellyfish are found in oceans and seas all over the world. They are found in warm, tropical seas and in icy water near the North and South poles.