In The Lord of the Rings Edit
Boromir is the son of Denethor, Ruling Steward of Gondor, and the older brother of Faramir. Had things gone differently, he would have assumed his father's place as ruler of the city and kingdom. However, things did not go differently.
Boromir joins the Fellowship of the Ring, stays with them until they reach Rauros Falls, and then tries to steal the One Ring from Frodo Baggins. A short time later, he gets turned into an oversized pincushion by either one (movieverse) or many (bookverse) Orcish archers. Then he dies.
In the books and the movies, Boromir is a noble man whose desire to save his city from destruction leads him to fall to the temptation of the Ring. He is not evil, and his death while trying to save Merry and Pippin from capture redeems his honor.
In Badfic Edit
Usually, Boromir is either a foam-brained Lust Object or a Designated Misogynistic Bastard. He is also occasionally slashed with his brother. The fact that he tries to forcefully take the Ring from Frodo means that most fanbrats see him as Teh Ebul!, much to Agent Acacia's annoyance.