As you probably know by now, contact dermatitis is a type of rash that occurs when your skin (derma) encounters a foreign substance and reacts badly.
Three types of contact dermatitis exist - allergic, irritant, and photo. The first is the most common, affecting 20% of people. Think of poison ivy or poison oak, or my experience with Mexican mango. Your skin has an allergy to a substance, so it reacts badly when you touch it.