This video describes how caffeine works. But what is caffeine? Caffeine (also spelled caffein) is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant drug. Caffeine was discovered in 1820 by a German chemist, Friedlieb Ferdinand Runge, and in 1821 by French chemists working independently; viz., by Robiquet and by Pelletier and Caventou. It was Pelletier who coined the word "cafeine," whence "caffeine".