Macbeth

The three weird sisters set the action of the play in motion when they confront Macbeth and prophesize that he will be King of Scotland. We never see them apart and they often speak and act in unison so it's worth considering them here as a single unit.

Ambiguity

From the play's beginnings, lots of ambiguity and drama surrounds these figures. When we encounter them in the play's opening scene, we're not sure where they've come from, who/what they are, or what they have in mind when they say they plan to meet Macbeth. What we do know is that they've gathered amidst thunder and lightening and move about the fog and "filthy" air, which seems just as murky and mysterious as they are. Even Banquo and Macbeth are unsure about the sisters' identity when they meet them on the heath. […] What are these
So wither'd and so wild in their attire,
That look not like the inhabitants o' the earth,
And yet are on't? Live you? or are you aught
That man may question? (1.3.1)

Appropriately, the weird sisters deliver the infamous lines that set the tone for the play: "Fair is foul and foul is fair" (1.1.4). In other words, nothing, including the identity of the weird sisters, is certain in this play.

Witchcraft

The play's subheadings and stage directions refer to the sisters as "witches," which makes a lot sense, given that they spend most of their time gathered around a bubbling cauldron, chanting, casting spells, conjuring visions of the future, and goading Macbeth into murder by making accurate predictions of the future (before they vanish into thin air, of course). The witches also do some interesting things with "Eye of newt and toe of frog, / Wool of bat and tongue of dog" (4.1.2). Do you notice the sing-song quality of the speech? The sisters' chanting sounds a lot like a scary nursery rhyme, which, depending on the attitude of the audience, can have the effect of making them sound a bit silly, despite their malevolent intentions. (See "Writing Style" for a discussion of how the sisters' speech sets them apart from other characters in the play.)

While the witches can, at times, seem harmless and even a bit petty (as when they cast a spell on a man after his wife refuses to share her chestnuts with one of them), they're often portrayed as evil forces with very real powers. You can read more about them by going to the theme of "Supernatural."

The Sisters and Fate

The sisters are called "witches" only once in the play, as opposed to being referred to as "weird" a total of six times. The term "weird," as we know, comes from the Old English term "wyrd," meaning "fate" so it seems pretty clear that they're in some way associated with the three fates of classical mythology. Why does this matter? Well, the "fates" are supposed to control man's destiny and one of the major questions in the play revolves around the issue of whether or not Macbeth's actions are governed by his own free will or by some outside force. It's possible that the weird sisters control Macbeth's actions and cause him to commit murder. On the other hand, it could be that they merely set things in motion and/or represent Macbeth's murderous ambition, which you can read more about by checking out "Quotes" for "Fate and Free Will."