A switch on the side of iPhone makes it easy to turn the ringer on or off.
Turn the ringer on or off:
Flip the switch on the side of iPhone.

Ring/Silent switch

NOTE: Clock alarms still sound even if you set the Ring/Silent switch to silent.

Set iPhone to vibrate:
In Settings, choose Sounds. Separate controls let you set vibrate for both ring mode and silent mode.

http://www.apple.com/iphone/tips/