Prince John is the younger brother of King Richard.  They are both African lions, though John, being generally weaker and scrawnier than his older brother, lacks a regal mane.  While he is, like his brother, a master of language and etiquette, Prince John is also a scheming, manipulative, greedy and megalomaniacal villain, as well as a cowardly weakling.  Because he is Richard's younger brother, he does not have any legitimate chance to become king unless his brother dies or is killed, so he has become insanely jealous and hateful of him.  Any mention of his older brother's name brings reactions which vary from insult, to fury, to babyish thumb-sucking and calling for his "mommy" (and later whining that "mommy always did love him best").  The public enjoys a shared private joke at Prince John's expense, taking great pleasure at mocking and imitating him.

Prince John, in a sinister and conniving move, convinced Sir Hiss to hypnotize Richard, making him believe that he had to take part in the Crusades.  King Richard left immediately, unintentionally allowing John the opportunity to usurp the crown, and the throne, out from under him.  John became a cruel tyrant, mercilessly over-taking the population to the very point of poverty, and lavishing himself (and his closest allies) with all manner of riches and comforts.  His cruelty and total lack of mercy earned him great hatred by the populace, and the hopes for a savior.  Which set the stage for the adventures of the rogue named Robin Hood.

Even though we learn, in the movie, that Maid Marian is the niece of King Richard, there is no direct statement in the movie (or the Robin Hood legends) that she is the daughter of Prince John (fortunately for her!).  However, she is probably John's niece as well.

Origins of the character: Unlike most of the characters in this movie (and the Robin Hood legends), Prince John was a real historical figure (as was his older brother, King Richard I, "The Lionheart").  John "Lackland" ruled England from 1199 until his death in 1216.  He was known as a pretty bad king, who did actually scheme to take his brother's throne from him (and failed), and was the very same King John who was later forced to sign and accept the famous Magna Carta.  He was also, interestingly, said to be rather weak and frail compared with his older brothers (as he had more than one), though his father Henry II did favor him over his other sons.  In the end, it didn't help John very much.