Although this question could be interpreted in a number of ways, I believe the best answer to be the difference between cause and effect. Are the witches truly revealing the future effect of a true prophesy in Act I when they say, "Hail to thee, Thane of Glamis! . . . Thane of Cawdor! . . . that shalt be King hereafter!"? Perhaps the witches are actually the cause of the whole problem. If they hadn't revealed the prophesy (and if Macbeth hadn't written that pesky letter asking his wife for advice about it), it is almost certain that Macbeth wouldn't even have entertained the notion of killing Duncan more than just a passing thought, especially when one considers the number of times Macbeth falters in his goal. Macbeth starts to hear and see things (like floating daggers, for example). He even lists in a long soliloquy the reasons why he shouldn't kill Duncan! Perhaps the key is in one of the witches' first statements: "Fair is foul, and foul is fair." What is truly exciting is that it can be argued either way.
In Elizabethan England, there were strong beliefs about witches, one of which was that witches were women who appeared to be men. When Macbeth and Banquo first meet the witches in Act One, Banquo expresses this idea by commenting on the fact that the witches are bearded, etc. So, Shakespeare plays on a popular belief at the time to enhance his theme of appearance vs reality.