I found this useful diagram at about.com. It shows the meaning of some theatrical terms: backstage, wings, orchestra pit.
Upstage is also used as a verb: to upstage someone. It means to steal the show from someone, to make them seem boring, dowdy or stupid by dazzling the ‘audience’. It comes from theatre language; as you can see from the picture, if an actor stands upstage from another player, then that one will have to turn his back on the audience in order to address him. And naturally, if an actor has his back to the audience when he speaks, then his words will not come across.
Example: When you are the mother of the bride, you want to stand out and feel special without upstaging your daughter.
A class act 