A jellyfish is a type of marine animal that belongs to the phylum Cnidaria. Known for their gelatinous, umbrella-shaped bell and long tentacles, jellyfish are found in oceans worldwide, from surface waters to the deep sea. They are primarily composed of water and have a simple body structure lacking a brain, heart, or bones. Jellyfish are known for their unique locomotion via pulsations and are equipped with specialized cells called nematocysts that help them capture prey and defend against predators. They play an essential role in marine ecosystems.