Available only on the iPad, the Guided Access setting limits users to just the current running app, which is great for a kiosk or as a form of basic parental control when you hand over an iPad to your kid. (Triple-click the Home button to enter Guided Access mode; triple-click it again and enter a password to exit.)
But Guided Access does more: You can disable certain areas of the app's screen, such as to prevent opening the iTunes Store within iTunes or accessing bookmarks in Safari. You just circle the area to block access to when setting up Guided Access. You can also disable the touchscreen entirely and/or disable screen rotation.