Who is macduff and how significant is he in the play?
Macduff is yet another noble who truly loved King Duncan. As your other answers tell you, he was first to suspect Macbeth for wrong-doing and Macbeth has Macduff's family murdered after Macduff flees Scotland to England and help with the army to overthrow Macbeth. Macbeth does this because the witches tell him to "Beware Macduff!." Macbeth already knew this since Macduff does not attend the state dinner Macbeth has after he becomes king (the one where Banquo's ghost appears), nor is he present at Macbeth's coronation.
Without Macduff, there would be less of a chance of Macbeth's overthrow. Shakespeare wants the audience to side with the English forces who are helping Malcolm back as rightful Scottish King. They need reasons to side with this army, and the murder of so many innocents helps achieve this.
It is also worth mentioning that by putting Macbeth down, the witches' prophecy of Banquo's issue becoming kings for many, many years is that much closer to coming true. The play was originally written for King James I of England (also King James VI of Scotland) whose ancestor is represented by Banquo.
Macduff discovers Duncan's death. He is possibly the first to be suspicious of Macbeth's quest for being king. He is noticeably absent during the banquet to honor Macbeth.
He is pivotal in Macbeth's undoing, because he was not "of woman born" but by cesarean, thereby not included in the witche's prophecy.
It is Macduff's family that Macbeth has killed because he is suspicious of Macduff, especially after Macduff failed to appear at the banquet in Macbeth's honor. Malcolm tells Macduff, then, to let his grief turn to a desire for vengeance against Macbeth. It is Macduff who kills Macbeth.