The term "cayenne" is often used to refer to any ground pepper, but true cayenne (which takes its name from the French Guinea city of Cayenne) is actually a particular type of chili pepper--about four to 12 inches long, thin, and very pungent. There are several scales for measuring the hotness of chilies. The most popular scale is the Scoville Organoleptic test, which assigns a number between 0 and 300,000 heat units. Cayenne typically measures between 30,000 and 90,000 heat units on the Scoville scale.
Chili Peppers, Canyenne 90K HU