It's easy to think that cayenne pepper is a pepper variety but that doesn't add up but it's a type of chili. Cayenne pepper is the small sharp fruits of a perennial low shrub. Like paprika, Spanish pepper, chili, etc., it belongs to the Capsicum genus. The potency of chili fruits is measured in so-called scoville units, indicating the amount of capsaicin, the chemical substance that makes up the potency. A mild chili variety can be between 100-500 scoville, medium-strong varieties between 1000-5000 scoville and really hot all the way up to 570,000 scoville. Cayenne pepper = about 27,000 scoville.
Cayenne pepper raises the heat in sauces, soups, dressings, seafood and fish stews, as well as in meat stews.