Capsaicin is what normally grabs the headlines when it comes to spicy peppers, though, it's only one member of a class of compounds called capsaicinoids. Capsaicinoids are what makes a chile pepper spicy/hot or what scientists prefer to call pungent. The higher the concentration of capsaicinoids in a pepper, the hotter it is. The hotness of a pepper is measured in Scoville Heat Units, or SHU. The higher the SHU, the hotter the pepper.
Capsaicinoids refer to a group of 22 known, naturally occurring proteins found in members of the capsicum family of plants. Of which, capsaicin (C18H27NO3), dihydrocapsaicin (C18H29NO3), and nordihydrocapsaicin (C17H27NO3) are the most commonly present. Nordihydrocapsaicin has about one-half the pungency of capsaicin and dihydrocapsaicin, while capsaicin accounts for roughly 50-70% of the capsaicinoids in a chile, depending on the variety.
The burning sensation capsaicinoids cause is not a function of our taste buds, but instead, the result of irritation to the transient receptor potential channel, or TRPV1. TRPV1 contributes to heat sensation, which is likely why it can trigger compensating bodily responses likely sweating. In addition to being present in the mouth, this receptor is also present in our skin and digestive system, explaining the various sensations the human body can experience when exposed to capsaicinoids.
Interestingly, only mammals are affected by this sensation, while birds and insects are believed to be immune. Capsaicinoid proteins are also antioxidant carotenoids and are believed by many to possess a number of health benefits. Unfortunately (fortunately for some?), capsaicinoids are not water soluble, so drinking water doesn't help if you find yourself in pepper overload. Though, it is soluble in either fat or alcohol.
Capsaicinoids are not evenly distributed within a pepper. Typically, the lower tip of the pepper has the least heat, while the white pulp attached to the seeds has the most.
The Scoville Scale was invented by Wilbur Scoville in 1912 to measure the pungency or heat level of a chile pepper. People commonly refer a peppers Scoville rating in SHU, or Scoville Heat Units. Originally, it was a subjective test that measured how much sugar water was needed when mixed with a pepper's extracted capsaicinoids to eliminate the heat. Being subjective and producing potentially high variability, High-Performance Liquid Chromatography, HPLC, is now used. Though, converting those readings back to the Scoville scale poses some issues and debate on the proper method and conversion factors.The scale ranges from 0 SHU (no heat, bell pepper) to 16 SHU million (pure capsaicin). Jalapenos clock in around 10,000 SHU, while habaneros are in the 100k-350k range. The title of the World's Hottest Pepper currently belongs to the Carolina Reaper with 2.2 million SHU. Curious to see where your favorite pepper ranks? Cayenne Diane has a comprehensive list of Scoville rankings.