iOS 6 included a cool new feature called "Do Not Disturb" mode which basically does as the name implies. It places your phone in a silent, no vibrate state where it can still send and receive messages but it won't alert you of anything. This is especially useful if you are in a meeting or if you have class and you don't want to be disturbed. Personally, I use this mode at night when I want to keep my phone in a sort of "alarm only" mode.