How does Macduff serve as a foil to Macbeth?

In literature, a foil is a character with qualities that contrast with those of a main character. These contrasts typically highlight the main character's strengths or weaknesses, particularly as those traits contribute to plot development.

Macduff serves as a foil to Macbeth in several ways. First, unlike Macbeth, he is a loyal and honest man, particularly when it comes to his king. It is Macduff who finds Duncan's body, and when he sees his king's bloody remains, he is so horrified that he can barely string words together:

Tongue nor heart
Cannot conceive nor name thee (2.3.67).

He remarks that the act of murdering a king is sacrilegious, since kings are ordained by God. Macduff's loyal reaction to Duncan's death thus serves to further emphasize the wickedness of Macbeth's murderous acts, demonstrating that Macbeth has not only acted selfishly—he has acted against God.

Act 4, scene 3, highlights Macduff's integrity. Macduff seeks out Duncan's son Malcolm, whom he believes to be the rightful ruler, to overthrow the tyrannical Macbeth. Malcolm, however, is unsure of Macduff's allegiances, and so he "tests" him, claiming that while Macbeth is bad ruler, he might prove even worse. As Malcolm details his various (pretend) vices, Macduff grows disgusted, and exclaims that Malcolm's wicked nature makes him unfit to rule:

Fit to govern?
No, not to live.—O nation miserable,
With an untitled tyrant bloody-sceptered,
When shalt thou see thy wholesome days again,
Since that the truest issue of thy throne
By his own interdiction stands accursed,
And does blaspheme his breed?

Macduff's horrified reaction proves that his intentions are honorable, prompting Malcolm to reveal his deception and agree to work with Macduff. Notably, while Macbeth is perfectly willing to commit treacherous acts to maintain power and satisfy his ambition, Macduff refuses to compromise his honor by swearing loyalty to an unworthy ruler. In refusing to support Malcolm—even though he is the rightful heir to the throne— Macduff displays a strength of character that is noticeably absent in Macbeth.

Ultimately, then, in showing how an honorable nobleman ought to act, Macduff makes Macbeth look even more selfish and weak in comparison. When news reaches Macduff that Macbeth has murdered his family, their role as foils and rivals is cemented, and a final battle between the two of them becomes inevitable.

In Macbeth, how does Lady Macduff serve as a foil to Lady Macbeth?

One way in which Lady Macduff can be seen as a foil for Lady Macbeth has to do with their gender.  During Shakespeare's time, gender roles were quite rigid and clearly defined: men were to be the protectors of their homes and families, the breadwinners and providers for those families, brave and strong.  Wives were to be ruled by their husbands, their behavior to be appropriately "feminine" and submissive.  Lady Macbeth is, clearly, not like this at all: she -- at least initially -- seems braver than her husband, and she is certainly not submissive to him; she takes the lead in planning Duncan's murder and has to help him keep it together after he does the deed.  Lady Macduff, on the other hand, is very maternal—a "feminine" and therefore expected quality—and she expects her husband to protect her and her family. When he does not, leaving them vulnerable to the tyrant Macbeth, she blames him and schools herself and her children to think of him as dead (as she believes it would be better for him to be rather than to be a coward).  

Further, both women die, indirectly, as a result of their husbands' abandonment of them.  When Macduff goes to England to find Malcolm, he leaves his family without any protection. Macbeth seizes this opportunity to punish him by murdering his entire family, wiping out his entire line.  Macduff doesn't kill his family, but his lack of foresight makes possible their murders.  Likewise, Lady Macbeth seems to become desperate after the murder of Banquo and the Macduffs; her guilt begins to consume her, and she eventually takes her own life, perhaps as a result of her terribly burdened conscience and consequent misery.  Her husband no longer consults her when making decisions, and he descends deeper and deeper into evil.  Neither Macbeth is happy; in fact, they both lament how sad it is to have what they thought they wanted, only to find that they cannot feel content.  Macbeth figuratively abandons his wife to her guilt and illness, just as Macduff literally left his wife alone; both, left alone to fend for themselves, die pitiably.  

Why is Macduff an effective foil for Macbeth in Macbeth?

Macduff and Macbeth serve as powerful contrasts with one another, not only in terms of personality but also in terms of their role within the play. From this perspective, they certainly emerge as foils.

For one thing, consider how both these characters exist as agents of prophecy, with the their fates ultimately intertwined with one another. We see this in act 4, scene 1, when Macbeth receives a second series of prophecies relating to his eventual downfall (prophecies he misinterprets to relate to his own invincibility). One of those prophecies states: "none of woman born / Shall harm Macbeth." As we learn at the end of the play, this particular prophecy relates to Macbeth's eventual defeat at the hands of Macduff, who was born by cesarean section. Just like Macbeth, Macduff's life is dictated by destiny. He appears fated to defeat Macbeth and to help restore law and order to Scotland.

However, this is not the only way these two characters are thematically linked (and contrasted) with one another. For example, consider that they both have similar roles for their respective lieges (Duncan for Macbeth and Malcolm for Macduff), but Macduff proves to be loyal to his monarch where Macbeth is treacherous, murdering his rightful king to usurp the throne for himself. In defeating Macbeth, Malcolm and Macduff bring an end to this tyrannical usurpation and restore legitimate monarchy to Scotland.

A foil is a character who exists to provide contrast.  Macbeth and Macduff are opposites.  Although both are brave soldiers and noblemen when the story starts, Macbeth becomes a murderer and a tyrant and Macduff vows to stop him.

Macbeth is the Thane of Glamis and Macduff is the Thane of Fife.  Both are important nobles to the kingdom, and both are apparently loyal to Duncan.

Macduff is everything Macbeth is not.  He is sensitive and emotional.  When King Duncan is murdered, he is horrified while Macbeth—Duncan’s murderer—only pretends to be.

O horror, horror, horror! Tongue nor heart
Cannot conceive nor name thee. (Act 3, Scene 3, p. 34)

He also asks people to look after Lady Macbeth when she seems to faint.  He is concerned with her health and safety even as the king is dead.  This is completely contrary to Macbeth, who thinks only of himself.

Macduff seems aware that Macbeth is up to something. He becomes suspicious.  Banquo says he suspects something.

Against the undivulged pretence I fight
Of treasonous malice (Act 2, Scene 4, p. 37)

Macduff replies: “And so do I.”  

Clearly, he suspects foul play.  It is not much of a step from there to suspecting Macbeth, in whose castle Duncan was killed.

From there, tragedy strikes. When Macbeth reaches out and has Macduff’s wife, son, and entire household killed, the two are fated to struggle to the death.  Macduff cannot let such an insult stand.

Malcolm, who has been feeling Macduff out to see if he really is loyal, tells Macduff to “dispute it like a man” and he says he will, but he still feels it.

But I must also feel it as a man.
I cannot but remember such things were,
That were most precious to me. (Act 3, Scene 3, p. 74)

Of course, Macduff is not the treacherous one.  He is determined to kill Macbeth in order to return justice and peace to the land.  He cares as much about his country as getting his revenge.

In the end, one foil often ends up killing the other.  Macduff kills Macbeth, the violent version of himself, because he has gone to the dark side.  He has turned evil.  Macduff feels responsible for taking him out.

Behold where stands
The usurper's cursed head. The time is free.
I see thee compass'd with thy kingdom's pearl  (Act 5, Scene 8, p. 90)

In Shakespeare's Macbeth, why is Macduff's plan effective?

I assume you are referring to Macduff's plan to leave Scotland at his earliest convenience and seek assistance for Macbeth's overthrow.

The plan is effective in the sense that firstly, Macduff, in leaving in such a hurry, denies Macbeth the opportunity to assassinate him as he had done with Banquo. Macbeth has been aware of Macduff's sentiments and knows where his loyalties lie and wants him dead. In fact, the murderers who kill Macduff's entire family come looking for him for, on their arrival, the first murderer asks Lady Macduff, "Where is your husband?"

The murder of Macduff's entire family and his servants indicates how ruthless and bloodthirsty the tyrant has become. With his rushed escape Macduff had, essentially, abandoned his family and left them vulnerable and open to Macbeth's malice. In this regard, the plan was not entirely effective. In his defense, though, one can argue that he had no choice but to leave. If he had tarried, he would have been killed as well. His escape ensured that he would be able to acquire support for what has become a desperate cause—the survival of his beloved Scotland.

Macduff meets up with Malcolm in England, where he is informed that Edward, the English king, has promised support for their cause in the form of "goodly thousands"—experienced and battle-hardened soldiers. Macduff also later learns that Siward, "with ten thousand warlike men, / Already at a point, was setting forth."

He and Malcolm eventually meet up with the troops as they approach Macbeth's castle from Birnam wood. They later overrun his castle, and Macduff is brought face to face with the evil tyrant. He kills him in a sword fight and decapitates him. The bloody tyrant has come to an ignominious end, and Macduff has had his revenge.