There is an ongoing trend to grow the hottest pepper one can, then eat, or at least taste it so one can (if you will) achieve bragging rights. Ignore, if you will, that effect the following day...
Now hot peppers contain a special ingredient: capsaicin. What can be said about capasaicin? This DecodedScience article provides an adequate answer and should anticipate most of your questions and more.
What Makes a Hot Pepper So Hot?
If you enjoy the piece, why not visit some of the author's other DecodedScience articles? They shouldn't be as painful as the after-effects of the hot peppers! Chocolate, coffee, and many non-food related articles are included.