Go to Settings > General
Go to Accessibility
Go to Guided Access
Enable Guided Access
This allows you to lend your iDevice. Whenever you want to lend your phone or table, triple tap the home button. This will let you enable the guided access mode, so they can just use one app.
Set a passcode. You will need it to enable and disable the guided access mode.
Triple tap the home button while any app is open, and lend your device. They won't be able to access any other app, because they won't be able to use the home button.
Triple tap the home button again to enter your passcode to disable the guided access mode.