Tweet
When you open an app on the iphone, it leaves the application open so you don't have to reopen it the next time you want to use it. This is a benefit of multitasking, one of the features of iOS4.
If too many apps are open, your iphone could suddenly shut down. You might also notice your battery draining faster than it used to. The more apps open on your iphone, the faster the battery drains.