In ancient Rome, Cupid was the incarnation of Eros, the god of love. As Eros, he was dark and full of lust and desire. Eventually, though, he evolved into the image we have today of a chubby cherub, flitting about zapping people with his arrows. In particular, he enjoyed matching people up with odd partners, and this eventually ended up being his own undoing.
Cupid, in his aspect as Eros, fell in love with a maiden named Psyche. Originally, Cupid was sent to destroy Psyche by his mother, Venus (the Roman version of Aphrodite), who was jealous of the mortal woman's beauty. Once he laid eyes on her, though, Cupid was completely smitten. Head over heels in love, Cupid arranged for Psyche to come live with him. However, he did this in disguise, so although she returned his love, she never actually saw what he looked like. Her sisters were so jealous of her happiness that they taunted her, saying that her new lover must be hideous indeed to hide his true appearance from her.
One night, Psyche's sisters convinced her to follow Cupid as he left her. He awoke and saw her standing there with a knife, and told her he must leave her, for "love cannot dwell where there is no trust." Realizing what a fool she was, Psyche went to Venus for her help. Venus, naturally, thought this was hilarious, and set about making Psyche perform a number of seemingly impossible tasks. In each case, Psyche was assisted by a god, and so she performed all the jobs assigned to her by Venus. Eventually, Psyche and Cupid were reunited, and allowed to marry.
Today, Cupid is seen on Valentine's Day cards and decorations, and is invoked as a god of pure love and innocence -- a far cry from his original form.