Key the home button twice.
Slide you finger all the way to the left.
See if the screen is locked.
You will see the picture, and it'll be clear for you, I am sure.
The Home button is the little round indentation at the bottom of your iPad --
Once you double tap this button, you should see some icons on the bottom of the screen. (These are the apps that you have used). When you drag your finger to the left across this bottom strip, you should see this --
The icon on the far right is the screen rotation lock and will indicated if it is locked.
Only if you are running an older version of iOS. When/if you update, the switch next to the volume becomes a mute toggle to turn on/off all system sounds and notifications.
After the update, the rotation lock is where 1991-C4 described, though the controls lay out is slightly different on my iPad.
The 4.3 beta update gives you the option to choose which of these switch does. Chances are this will still be there when the update is released, but no guarantees. Apple really does mean beta features are beta, and has pulled them before for various reasons.