He is with Macbeth during the encounter with the witches near the beginning of the play. After predicting that Macbeth would be king, they predict that Banquo would never himself be king, but would beget a line of kings (the Stuart line of Scottish and English kings). Macbeth later sends murderers to kill him and his son Fleance. Fleance escapes, and the ghost of Banquo returns to haunt Macbeth.
Although Macbeth was a historical figure (a very different one from the character in the play), Banquo was not; he is an invention of Shakespeare's. His "descendant", James I, had just been made king of England in addition to Scotland, creating the United Kingdom.
wikipedia.org dumped 2003-03-17 with terodump