An interesting bit of etymology about the word "weird" is that it comes from the Old English word "wyrd" meaning "fate." The witches are very similar to the Fates in mythology who shared an eye and could see the future for mortals. They cut the threads in the tapestry of life when they were supposed to, but had no control over the actions of the humans who had free will.
Similarly, the witches in Macbeth share information that can be interpreted in multiple ways (the apparitions who tell him he can not fall until the forest marches up the hill, that no man born of woman can harm him, and that he must beware Macduff). They have no real control over Macbeth since he has free will, but they know that giving him a false sense of security will cause him to make erroneous choices. They enjoy toying with mortals (for example, the sailor's wife who refused to share her chestnuts), but Hecate and those who call her, "Hark! I am call'd" (3.5), are the ones with the real power.
The Weird Sisters are the three witches who make predictions regarding Macbeth's future. In the beginning of the play, they tell Macbeth and Banquo that Macbeth will be thane of Glamis, thane of Cawdor, and king. Then they tell Banquo that he will be the father of kings. They vanish without answering any questions, but Macbeth can't get them out of his mind because they have presented him with the idea of taking the throne by killing the present king. Their evil lies in their ability to taunt Macbeth with possibilities of what could happen if Macbeth is willing to make it happen. The witches don't have the power to make it happen, however. Macbeth must give in to his darker evil side for the events to happen. Macbeth realizes too late that the witches have misled him with their words.