What important roles does Macduff have in Macbeth?

Macduff plays a number of important roles in Macbeth.  Throughout the play, he acts as one of the primary foils to Macbeth himself. A foil is a character who exhibits qualities that contrast with those of the main character in order to accent the qualities of the main character. In the play, Macduff is a stalwart, loyal, honorable man; this is in direct contrast to the treachery and cowardice of Macbeth.

The first point at which the reader sees Macduff as a foil occurs in Act 2, Scene 3, shortly after Macbeth has murdered King Duncan. Macduff arrives at Macbeth's castle and discovers the body. He laments the loss sincerely, unlike Macbeth, who puts on a show of false sadness and rage. As the play continues, the actions of Macbeth and Macduff are juxtaposed repeatedly. When Macbeth is to be crowned king, Macduff refuses to go to Scone to see the coronation. After Macbeth has had Banquo killed and throws a dinner party for the other Scottish lords, Macduff doesn't attend. Instead, Macduff goes to England to enlist the help of Malcolm, Duncan's son and the rightful heir to the throne, in an effort to overthrow Macbeth. At the aforementioned party, Macbeth notices Macduff's absence, and begins to grow more paranoid about threats to his reign. Shortly after, he consults the Weird Sisters, who conjure an apparition that tells him to "Beware Macduff" (4.1, 81). This causes further paranoia, and Macbeth orders the murder of Macduff and Macduff's entire family. 

When the murderers arrive at Macduff's castle, Macduff is still in England. He is saved, but his entire family is murdered. When news of the horrible tragedy reaches him, Macduff demands vengeance. He returns to Scotland with Malcolm and the army that Malcolm has mustered, and together they storm Macbeth's castle. Macbeth's own men begin to flee or fight against him, and eventually Macbeth finds himself face to face with Macduff. The two fight, and Macduff slays Macbeth, taking his head back to Malcolm at the end of the play.

Macduff has a number of important roles throughout the play. He is one of the first to suspect Macbeth of wrongdoing. His loyalty and love for Scotland are one of the major catalysts for the end of Macbeth's reign, and near the end of the play, Macduff is the only man who is able to locate and slay Macbeth.