Jellyfish are not actually fish at all, instead being of the phylum Cnidaria, some of which are specialized cell structures which work together instead of forming a true single organism. Jellyfish are extremely common, found in every ocean in the world, and are very simple creatures compared to more complex marine life.
Because of their simplicity, jellyfish do not have many specialized organs. Their respiration, for example, is accomplished through skin diffusion rather than gills and lungs, and they have no central nervous or circulatory systems. They also have no eyes, instead sensing light on the surface of their dome and reacting to external stimuli entirely through a loose "nerve net" covering the body. Jellyfish generally swim by taking in and expelling water, and have specialized cells that form tentacles for catching and killing prey.
Because of their reduced need for oxygen, jellyfish can expand to fill low-oxygen areas where other predators can't live; they can also live in oceans with higher concentrations of salt, which allows their reproductive polyps to grow faster. However, there is no single habitat where jellyfish come from; they grow and flourish anywhere in the oceans, even the extreme cold of Antarctica.