Jellyfish belong to the class Scyphozoa that is broken down into two orders, Rhizostomeae (no tentacles on the fringe) and Semaestomae (tentacles on the fringe). In the aquarium trade, members of the families Cassiopeidae and Ulmaridae are the most common. Jellyfish are found in most temperate marine environments. In the wild, the jellyfish can be found as individuals or in large schools floating at the surface. Jellyfish are 90% to 95% water, but a few species are still eaten by other predators. Suspended beneath the umbrella or top of the jellyfish is the manubrium that connects the mouth to the stomach. The oral arms are sometimes joined together to form a basket-like structure used to stir up and draw in plankton.

Jellyfish have two distinct stages of morphology. One stage is called the scyphistoma in which the small polyp is anchored to a substrate and can multiply asexually. The scyphistoma will eventually turn into the scyphomedusa that is the familiar, floating jellyfish, which can reach a size up to 6 feet in diameter. (For the record, the Portuguese Man-O-War is not a jellyfish but a colonial cousin.) The scyphomedusa reproduces sexually and sometimes will practice brood care, looking after the young.

One distinguishing characteristic of jellyfish is the formation of nematocysts. These nematocysts, or stinging cells, are expelled to capture prey and defend the jellyfish. While some nematocysts are merely a modified harpoon, others contain toxins to stun or even kill the target.

Members of the Cassiopeidae family are mostly bottom dwelling, and are often found lying upside down on the substrate. For these jellyfish, gentle currents will be required to provide movement in the aquarium, and foods designed for filter-feeding invertebrates should be offered. These jellyfish also contain the symbiotic algae zooxanthellae and require sufficient light to provide food to the organism. Larger, upright aquariums are ideal for jellyfish from the Ulmaridae family.