In Romeo and Juliet, what does Juliet mean when she says "That which we call a rose / By any other word would smell as sweet"?

As she stands on her balcony in Act 2, Scene 2, Juliet is trying to reconcile in her mind the fact that the man that she has just experienced love at first sight with is none other than a Montague, and, specifically, the son of her family's great enemy.

In her musings, she mentions that it is Romeo's name (Montague) and not Romeo himself who is the enemy.  It is then that she utters the next lines:

What’s in a name? That which we call a rose                                   By any other word would smell as sweet.                                        So Romeo would, were he not Romeo called,                               Retain that dear perfection which he owes                              Without that title. (Act 2, Scene 2, lines 43-47).

Juliet is making an analogy here.  Essentially, she is saying that names are meaningless.  A rose would smell sweet whether we called the object a "rose" or a "thumple" or a "gobbeldythwacker" (I am making up these names here -- Juliet didn't come up with these!).  Likewise, Romeo would be just as perfect even if his name were not Romeo Montague.  

In the lines that follow, Juliet states that she wishes that Romeo would trade in his name, and that he could have her instead.  It is at this point that Romeo reveals he has heard this, and that he has been hiding in the bushes nearby all along.