What are 3 classes of Cnidarians and how are they different from each other exactly?

There are four classes of cnidarians. Syphozoans, Hydrozoans, Cubozoans, and Anthozoans. Anthozoans don't have a medusa stage of life, so they don't get credit for being a jellyfish because they are never free swimming medusae. Syphozoans are considered by scientists to be a 'jellyfish', but the other two classes are considered jellies also (with reservations) because even though they have two layers of epithelial tissue with mesoglea (non-living gelatinous secretion) between the exodermal and the endodermal layers of 'skin', they are not radially symmetric. Other invertebrates of the gelatinous marine environment include the ctenophores and scalps.

Scyphozoans are characterized by their radially symmetric, umbrella shaped bodies.

Hydrozoans are characterized by their colony of polyps, which are not radially symmetric.

Cubozoans are characterized by their four sided, roughly cubical shaped bodies. They are also characterized by their 24 eyes, ability to perceive their environment, and actively both evade predators and pursue prey. These guys are not drifters, like most of the members of the 'jellyfish' distinction.