A genus of peppers, which contain capsaicin which creates the burning sensation on your toungue and in your mouth... So the stuff that makes them hot while the Red Savina Habanero is the hottest pepper in the world and tops the Heat Scale for peppers at around 350,000-580,000 Scoville Units the scale used measure the heat of peppers(Jalepeno Peppers range from 2,500 - 5,000 Scoville Units as a reference)
Pure capsicum(capsaicin) tops this scale far above the hottest peppers sitting atop the scoville heat scale at 10,000,000 units it is inedible for most people and they now use it to make the world's hottest hot sauces which people usually will buy to say they own it while never actually eating it most of these hot sauces will have instructions and on them reading "CAUTION Not to be directly ingested only to be used as an ingredient in cooking" and so forth, so you can guess that it is quite hot, spicy, and dangerous.
The world's hottest sauce I know of is made by Dave's Gourmet, it's called "The Source" made from diluted Capsicum(capsaicin), sitting around 7.5 million scoville units on the heat scale. Go try some yourself, if I ever get a chance maybe I will