Pepper spray is a generic term for all self-defense sprays that contain oleoresin capsicum, a carrier, and a propellant. Oleoresin capsicum is the active ingredient in pepper spray, harvested or derived from cayenne peppers or other peppers. Its effects are physically incapacitating rather than painfully irritating; this makes it ideal for self-defense sprays that are intended for use on assailants who are immune to pain (psychotics, those under the influence of drugs or alcohol, or just tough guys).
A Guide to Pepper Spray Laws
Pepper spray is the most popular self-defense weapon, but it is not legal in all states. At the time of publication of this writing, common pepper spray law was that you are allowed to carry it in all states.