Lady Macduff is a character we see very little of in Shakespeare's Macbeth. In fact, she only appears once, in the scene in Act IV in which she is killed. Lady Macbeth is a much more important figure to the plot of the play and to its themes.
Lady Macbeth is a powerful woman who has influence over her husband, a noble thane and military leader, Macbeth. When he hears the prophecy that he will become king, he writes to his wife immediately. He seeks her counsel and trusts her opinion. The two plot together to kill King Duncan when he comes to stay at Macbeth's castle so that Macbeth can ascend to the throne as soon as possible. Though Macbeth has some doubts along the way, he ends up going through with the murder. His wife is the stronger character at this point in the play: she is the one who convinces him to commit the crime when he tries to back out, and she is the one who goes back to plant the daggers on the guards to frame them, since Macbeth is too nervous and horrified to return to the scene. It is Lady Macbeth who has to cover for her husband when he thinks he sees Banquo's ghost at the table during a feast at this castle to celebrate his coronation. Soon enough, though, Macbeth takes total control, becomes increasingly ruthless and paranoid, and begins to leave his wife out of his plans. She ends up going insane, sleepwalking, and trying to wash her hands of a blood spot that symbolizes her guilt. She commits suicide near the end of the play, and her death makes Macbeth soliloquize about how brief and meaningless life is.
Lady Macduff, on the other hand, is more a victim than an active player in Macbeth. She is seen conversing with her son and with another nobleman about the sudden departure of her husband Macduff, who has gone to fight with Malcolm to overthrow Macbeth. She tries to explain to her son that she thinks Macduff is a traitor for going against his king, though Ross attempts to assure her that he has his reasons. Lady Macduff tells her son that his father is dead (he is not) and dramatically asks what they will do without him. She seems to be emotional and distraught, unlike the very much in-command Lady Macbeth, at least early in the play.
As the earlier reviewer said, one similarity between the two women is that they do critique certain actions or qualities of their husbands. Lady Macduff is upset because Macduff has left the family and the castle unguarded; this turns out to be a legitimate fear when everyone in the castle is murdered by Macbeth's henchmen. Lady Macbeth worries that her husband does not have the viciousness required to kill The King and take his position. She is partially correct, as well, since at the time of the murder, Macbeth is overwhelmed and weaker than she is. Eventually though, Macbeth proves her wrong.