The capsicum family includes peppers from temperate to eye-popping; the more capsaicin, the hotter the pepper. Smaller peppers tend to have more zing because of the seeds-and-veins-to-flesh ratio. Capsaicin, while concentrated in peppers' seeds and veins, is present to a lesser degree in the fruit's flesh.
A pepper's Scoville rating, named for pharmaceutical chemist Wilbur Scoville who devised the classic heat test, provides a temperature guide. Higher numbers are hot, hot, hot.
Thai, Scotch bonnet, African birdseye, some habaneros
Cayenne, piquin, tabasco
Serrano, aj amarillo
Ancho, guajillo, pasilla
New Mexican, poblano
Sweet bell, pimiento, U.S. paprika