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.
Note: Clock alarms still sound even if you set the Ring/Silent switch to silent.
Set iPhone to vibration mode: In Settings, choose Sounds. Separate controls let you set vibrate for both ring mode and silent mode.
Switch between ring and silent mode: Flip the Ring/Silent switch on the side of iPhone.
Source:
http://www.top10iphonedownloads.com/iphone-ringers-settings-and-silent-mode-ring-switch.php