Hold down the On/Off button and the Home button at the same time for at least ten seconds. Release both when the Apple logo appears.
You won’t lose your photos, apps or settings. It is just like doing CTRL-ALT-DEL on a PC.
If you have upgraded to IOS7 recently, you’ll have to do this at some point when you start to see Facebook crashing or the camera refusing to take pictures. Once you’ve reset, all will be well again.