Of course, you should probably already know this one by now, but it’s gotta be mentioned since it’s another one I constantly use. Just double tap the Home button while you’re in any app and the multitasking bar will pop up at the bottom.
Then you can switch to another open app by just tapping it. Very useful than exiting the app and then finding the other app and then opening it.
"iPhone Tip – Multitasking"
Reference : online-tech-tips.com