The disguises and tricks of Portia and Nerissa illustrate the theme of appearance versus reality in The Merchant of Venice. In reality, Portia is a woman, and so, by the standards of her time, she is incapable of defending a man in a court of law. Yet by disguising herself as a man, Portia is, ironically, able to showcase her real talents as a lawyer. Shakespeare seems to be saying that sometimes it takes appearing as something we are not to show who we really are.

Portia and Nerissa, disguised as a lawyer and a clerk, also insist that Bassanio and Graziano give them rings they received from Portia and Nerissa that they promised never to part with. The men give up the rings reluctantly and only out of deep gratitude to the twosome for saving Antonio. However, they are once again deceived by appearances, for unbeknownst to them, they are giving the rings back to their beloveds.

Antonio suffers from the difference between appearance and reality. He seems both prosperous and secure with three ships out to sea, and so he doesn't worry about the loan he has taken out with Shylock. A grim reality appears for a time to take over when all his ships seem lost at sea: but as with the rings, reality is not as it appears, and Antonio's ships do come to port.

Shakespeare depicts reality as a slippery business, full of twists and turns that can deceive, though in this comedy all works out well in the end, at least for the principal Christian characters.

I would add to the excellent answer above that Portia's decision to disguise herself as a man in act IV of Shakespeare's The Merchant of Venice can illuminate even more about the theme of appearance vs. reality in the text. Portia's donning of the disguise is an expression of this theme made physical: she is not what she appears.

Portia takes on this disguise so that she will be able to defend Antonio, her husband's dear friend, in a court of law. That she must pretend to be a man to do so is symptomatic of society at the time. Portia is just as intelligent and capable as "Balthazar" (her alter ego), yet only Balthazar is taken seriously.

Portia uses her necessary deception to her advantage by testing her husband's loyalty to her while interacting with him as someone else. Act V of the play, after all of the Shylock drama has concluded, deals largely with the aftermath of Portia and Nerissa's deception. Both women use their knowledge of their husband's actions while in Venice—which their husbands don't know they know—to their advantage. The comedy in the scene stems from dramatic irony. The audience, Portia, and Nerissa all know that things are not as they appear.

In The Merchant of Venice, the theme of appearance and reality is prominent because many of the characters are not what they seem and their actions are sometimes contradictory to their seemingly honest motives. Furthermore, the caskets that hold the secret to a successful marriage to Portia mislead her suitors and they choose poorly.

1. Shylock appears to be willing to loan money to Antonio even though he "hates him for he is a Christian" (I.iii.37). However, the penalty if Antonio cannot repay him is unreasonable and whereas it is not taken literally by the Christians, it is meant literally by Shylock when he demands his "pound of flesh."

2. Antonio appears to be virtuous and generous. He loves his friends and has a good reputation. Yet he treats Shylock very badly simply because he is a Jew which makes him a hypocrite. Shylock reminds the reader that it is Antonio's double standards that have led to this situation. Shylock refers to "the villainy you teach me" in Act III, scene i, line 61 meaning that Christians like Antonio are responsible for the animosity (hatred) between them.

3. Portia's suitors must choose from three caskets of gold, silver and lead if they desire to marry her. Portia is relieved that most of her suitors to date have chosen foolishly as they have been misled by the words which describe the contents of the caskets. Only one has Portia's portrait inside but so far the suitors have not realized that there is a warning contained in the words that read "All that glisters is not gold" (II.vii.65). Only Bassanio will choose wisely and there is still debate as to whether Portia guides his choice (which would be forbidden by her father's will) or whether he makes his own considered choice.

4. Even Jessica fools her father and runs away with his precious ring, selling it and therefore revealing that she has no respect for Shylock. 

5. The actions of the characters have been quite selfish and the theme of appearance versus reality has shown these traits but Portia will show great "mercy" and compassion when she disguises herself and helps Antonio avoid what would have been certain death if he had allowed Shylock to take his bond of a "pound of flesh." 

Shakespeare then reveals the theme through character development and this helps drive the plot and subplots forward.