Cupid (known in Greek as Eros) is the Olympian God of love and passion whose main duty is to inspire love in others by shooting them with his magic arrows which can make anyone fall in love with the first person they see.
In Hercules, Cupid appears as a tall, teenage God with fuchsia colored skin and angelic wings, but in the animated series, he is portrayed as a short, overweight, pink-skinned man with tiny wings and wearing a diaper. He also has a thin mustache and a gap in his teeth. A theory for this could be that he, like his family the Olympians, is known to shapeshift, and if that's the case, the animated series version could be a transformation, and the film version of him could be his true form.
In "Hercules and the Long Nightmare", it was revealed that originally Cupid was originally intended to be the God of War while Ares was going to be God of Passion, however due to Cupid's inability to even pick up a sword and Ares' violent nature, Zeus changed his mind and switched their positions thus giving Cupid his true purpose.
In "Hercules and the Bacchanal", when Hercules needed to raise Phil's Island from the bottom of the sea after Poseidon sunk it he made a deal with Poseidon to bring him some eye cream in exchange for raising the island, in order to do this he visited the multi-eyed monster Argus Panoptes who lives at the foot of Mount Olympus as he possesses large quantities of the cream, but Argus wants one of Cupid's love arrows so he can get a date for Saturday night. Cupid, however, wants to exchange his arrows for something that will help him forget Aphrodite's annoying theme song so Hermes suggested to give him some Lethean water from the Pool of Forgetfulness, and through a series of misadventures Cupid finally receives his water and forgets Aphrodite's cursed theme song, sadly he also forgets and realizes with some shock and humiliation that he wears a diaper.
Cherubs are small angelic beings who aid Cupid in spreading love. They also act as servants to the Gods on Mount Olympus. They are all infant-like in appearance and they tend to speak in a whimsical manner. Cupid himself is a putto.