When you close an application, iOS only limits the app to run 7 things in the background if it needs to:

1. Background audio - application continues to run in the background as long as it is playing audio or video content[28]
2. Voice over IP - application is suspended when a phone call is not in progress
3. Background location - application is notified of location changes
4. Push notifications
5. Local notifications - application schedules local notifications to be delivered at a predetermined time[28]
6. Task completion - application asks the system for extra time to complete a given task[28]
7. Fast app switching - application does not execute any code and may be removed from memory at any time[28]
Any applications that uses the features highlighted in bold above will use 3G or Wifi.

VoIP is used for phone calls

Push notifications are like when you receive messages / notifications from SMS / Apps.

Voice over IP will be applications like Viber and Skype. So when you do a Viber or Skype call, you can press the home button to close the application and still continue talking on the call because the call is still running in the background.

Push Notifications would be the notifications you get from apps like Facebook (for example) to alert you a friend has added you on Facebook, or posted a comment. Another example of an app is Whatsapp, notifying you that someone sent you a message.

For background location services, an example would be the Reminders app. For example, if you set a Reminder to remind you to buy milk when you leave home, then the Reminders app will constantly look at your GPS location even when the Reminders app is closed. Once it detects from your GPS location that you have left the house, it will send you a notification reminder.

Hope this helps!

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Added on December 19, 2011, 3:22 pm

Haha no worries if you have a data plan. If you don't, wah then every 1 kb is abit painful.