Who is Brabantio, and why do Iago and Roderigo awaken himin the middle of the night?

Brabantio, in Shakespeare's Othello, is the father of Desdemona, the woman whom Othello has just married at the beginning of the play. Brabantio also happens to be a Venetian senator. Iago explains in the first scene that he, Iago, "doth hate the Moor [Othello]" and is resolved to punish him for having established Michael Cassio, "who never set a squadron in the field," as his lieutenant, while Iago must be content with being "his Moorship's ancient." He wakes Brabantio in the middle of the night to create a scene, telling him that "an old black ram is tupping your white ewe." He wants to horrify Brabantio with the realization that Othello has just married Brabantio's daughter without his knowledge, and thus have Othello punished for this.
Roderigo is happy to join Iago in this matter because he is in love with Desdemona, so if Othello were to be punished and the marriage potentially declared invalid, he might have a chance of getting what he wants. Indeed, when Brabantio learns what has happened, he declares that he would rather Roderigo had married Desdemona; while he clearly had little love for Roderigo before, the idea of him marrying Desdemona is less unpalatable than that she should be married to Othello, against whom he has a racial prejudice.