Pepper spray is a natural derivative of hot peppers. It is the the most popular, the safest and the most effective defensive spray. It is much better than the old Mace or other chemicals like oven cleaner. The pepper essence is extracted, combined with a carrier and propellant, and put into a spray canister.
When it is sprayed into the eyes and face on an attacker, it is extremely painful, but it does not rely on pain for its effectiveness. The pepper chemical, OC, (Oleoresin Capsicum) causes physical effects even in people who are "feeling no pain."
It makes the eyelids slam shut or spasm, causing hampered vision or even temporary blindness. When even a small amount is inhaled, it constricts the bronchial tubes, causing extreme choking and coughing, though it does not cut off breathing. It is usually effective on most people and animals, including the mentally ill and those under the influence of drugs and alcohol.
Pepper spray comes in various concentrations, and various degrees of heat (measured in Scoville Heat Units, or SHUs). If you're going to get OC, you should purchase a product which has an SHU rating of 1 to 2 million or more. A 1% to 5% concentration is the best range. Sometimes concentrations higher than 5% do not atomize well, and are therefore less effective; also, they lengthen the recovery time beyond the usual 45 minutes or so, and there is no advantage to that in personal defensive situations.
Pepper spray has been used by police and federal law enforcement organizations for decades, and it has an excellent safety and effectiveness record. It is probably the best "general purpose" self-defense tool that you can find, though nothing is 100% effective. The spray may not work on some violent, goal-oriented, people, and may not work fast enough to be effective against someone who is armed.
That depends on your locality. The legality of these products runs the gamut from anyone can buy it, to, as in Massachusetts, you used to need need a license to purchase and possess it, to some locations where currently it's illegal no matter what. (Massachusetts law has changed. As of Jan 2015, no license is required!) It is illegal under FAA regulations to have in carry-on luggage on any airplane, but it is OK to have it in checked baggage. Check the FAA site to see current regulations.
Also, be sure that it is legal to possess in your state and your destination. Although unofficial, the following sites have a good summary of state regulations. As always, the individual state laws are the ultimate authority.
This depends on the laws in your area. It can vary from the hardware store to a firearms store to a supermarket. Check the Internet for a firearms, sporting, police supply store, or safety products store.
What Brand Shoud I Get? What Size Cannister? What Type of Spray?
See AWARE's blog on Choosing Pepper Spray.
How Do I Use It?
Proper use of OC is not like hosing a bug down with a can of bug spray! Most failures of OC to effectively control a situation stem from the inappropriate application or use: too near, too far, or too much spray used. See AWARE's blog on Using Pepper Spray for more details.