A stimulant is a drug which produces an sense of euphoria or awakeness. Examples of stimulants are caffeine and amphetamines. Stimulants are used both as recreational drugs and to improve productivity.
Curiously, antidepressants are not stimulants in that they generally have no immediate effect on mood.
Recently, there have been improvements in the area of stimulant pharmacology, producing a class of chemicals known as "eugregorics", or "good arousal". These stimulants tend to increase alertness without the peripheral (body) effects or addiction/tolerance/abuse potential of the traditional stimulants. They have minimal effect on sleep structure, and do not cause rebound hypersomnolence or "come down" effects. Currently, there are two stimulants in this class being used: Modafinil and Adrafinil, marketed as Provigil and Olmifron, respectively.