Multitasking is the biggest feature introduced in the iPhone OS 4.0. It allows you to run multiple applications at the same time. Many people have wanted this feature for a very long time and many of the newer phones already have this feature.
Steve Jobs said:
We weren’t the first to this party, but we’re going to be the best.
So far from my experience, multitasking is awesome but requires developers to update their applications for certain features to work. Pandora does not work currently. They need to push out the update to the AppStore before we can test all of the multitasking features.
Every time you close an application it stays running in the background. until you double tap the home button to bring up a new dock with all your running applications. To completely shut down an application you have to press and hold on the application until you see the red minus badge to delete.