Endorphins are your body's
natural painkillers.
Your body's pain transmitters, incidentally, are really pretty clumsy, and don't pay too much attention to subtlety-- they just kind of run full blast, caring about nothing but just getting the signal through. This isn't a problem at all because of the endorphins-- endorphins are chemicals that sit around in the gap between the pain transmitters, get in the way, and muffle pain. You have a steady baseline of this stuff that keeps things like touching your keyboard from being unbearable, and whenever serious pain starts to happen for some reason-- say you scab your knee-- your brain gives off big concentrations of the endorphins to keep the pain from being counter-productive.
Opiates (morphine, heroin, popium, codeine, methadone), the worlds' most effective painkillers, are effectively pseudo-endorphins; they fake the structure and function of endorphins, only on a much grander scale, and the body reacts as if they were real endorphins. Endorphins are named after morphine and not the other way around because morphine was discovered first. (see "endo-")