For many iPhone owners, multitasking was the biggest missing feature. iOS 4 aims to solve this problem. In iOS 4, tapping the Home button to close an app now only hides the app's activity. To see your open apps, you simply double-tap the Home button--the current screen pushes up to reveal a dock. You can swipe left or right to browse through the open apps. To close an app or task, you tap-hold on an app.
If you are reading e-mail and you want to listen to Pandora in the background, for instance, you tap the Home button once, open Pandora, and then double-tap the Home button and tap on Mail to return where you left off.
The problem right now is that the majority of apps are not multitasking-enabled, but in the coming weeks you can expect developers to update the bulk of these apps to add multitasking support.