the new iphone user guide, including iOS 6 is available in the ibook store. i suggest downloading it, as it covers the purposes for each of the settings, including Do Not Disturb.
when someone calls you, and do not disturb is active, the phone will not ring and the call will be directed straight to voice mail.
when someone sends you a message, the message will come through, but the phone will not indicate that you have received a new message.