The witches are the three sorceresses who, on the heath, tell Macbeth that he will be king. The play also calls them the Weird Sisters, and a fourth figure, Hecate, appears as queen of the witches.
They open the play (Act I, scene 1) by arranging a meeting: they will meet with Macbeth there. In scene 3, they greet Macbeth as Thane of Glamis, Thane of Cawdor, and future king, and tell Banquo that his sons will be kings, though he will not be. Then they vanish, and Ross and Angus arrive and confirm the title of Cawdor.
On the heath, the witches do not order anyone killed: they say what will be, and it is Macbeth who thinks about murder. In Act III, scene 5, Hecate rebukes them for dealing with Macbeth without calling her, and promises illusions that will lead him to scorn Fate, because excessive confidence is man's greatest enemy.
In Act IV, scene 1, in a cave, they prepare a cauldron, and Macbeth comes back to seek them out. Three apparitions appear: a helmeted head, which tells him to beware Macduff; a bloody child, which tells him to be bloodthirsty, fearless, and resolute, and that no man born of woman can harm him; and a crowned child holding a tree, which speaks of Birnam Wood. He also sees eight kings, with Banquo at the end. After this, the witches do not return to the scene, and in Act V Macbeth accuses the demons of playing with words.