An exoskeleton is an external alternative to the skeleton of most vertebrates. All arthropods (insects, arachnids and crustaceans) have exoskeletons as do many other invertebrates and some species with internal skeletal structures also sport exoskeletons.
Like a skeleton exoskeletons support the body but also often work as a from of armor.
Most (if not all) insectoid species possess exoskeletons, as do the Ascendants, the Tholians and the Lirin. (DS9 novel: Rising Son; ENT episode: "In a Mirror, Darkly, Part II"; EV comic: "Our Dearest Blood") The Lirin exoskeleton was composed of chitin. (EV comic: "Our Dearest Blood")