When your device is connected to WiFi network the data connection (via 3G) is not used by the system, however, it still remains enabled and increases the battery usage. Especially when your device is connected to your home or corporate network for a long time.
Add the application widget to your screen, and the app will start working immediately. The widget will show the currently used data connection.
Tap on the widget to switch it between “Auto” and “Manual” modes. In “Manual” mode the application is disabled and you may turn your connections on and off as usual. Put it into “Manual” mode when in roaming to prevent automatic enabling of 3G data connection.