Did you ever want to demo an iPad app without the users being able to leave it?

Starting from iOS 6.0 it's possible to put the iPad in kiosk mode. You can achieve this by turning on Guided Access.

Guided Access settings can be found under Settings -> Accessibility -> Guided Access.
You can enable this feature here and set a passcode to allow users to turn it off again.

By default, Guided Access can be started by a triple click on the iPad's home button. If it's the first you start this option then you'll be asked to set a passcode that the user will need to be provide when trying to exit Guided Access.

After doing a triple-click on the home button, press the start button to enter Guided Access.  The current user will not be able to exit the app unless entering the correct passcode.