Pepper Spray is a self-defense tool made up of oleoresin capsicum, which is derived from the chile pepper. It comes in many forms, such as aerosol spray, fog, foam, or gel. It is also available in very discrete carrying forms, such as the stunning ring, pepper pen, or pepper pager. Unlike most forms of self-defense, pepper spray does not actually injure the person or creature that you are defending yourself from, but rather renders them immobile with pain. You can use pepper spray on another human or an animal as it is effective with both. This is what makes it so reliable. Have you ever tried to use martial arts on a bear? It tends to end badly.
Pepper spray works by inflaming the mucus membranes of the attacker and causing the eyes to tear up, and it also makes it hard for the attacker to breathe. This reaction lasts about 30 minutes; long enough for the person who is being attacked to escape and/or get help. Unlike other forms of self defense, pepper spray has no permanent lasting effects on the person who has been sprayed.
Aerosol spray cans are the most popular form of pepper spray. This is because it is convenient to clip on to a purse or wallet or set of keys. The down sides of aerosol sprays are that aerosol tends not to travel as far as streams, foam or gel. Also if it is a windy day, there's a good chance the wind could carry your spray in the wrong direction.
If you don't want the chance of the wind blowing it off your target, your best bet would be to purchase pepper foam or gel. It's not as convenient as aerosol in the fact that it takes a few seconds longer to affect your attacker's mucus membrane, but it is more accurate and it also shoots further because it is heavier. With aerosol it is more of a mist. The foam and gel are more solid and stick together, making it harder to get off the face of the attacker. Pepper foams and gels sometimes come with UV dye in them as well, which makes it easier for law enforcement to capture the attacker.