Here is a summary of how to use the Control Centre in iOS (9.2) to choose where your device gets its internet connection from - wifi or cellular (SIM card). The Control Centre is accessed by swiping upwards from the bottom edge of the screen when viewing the lock screen, home screen, or from within apps (depending on your device’s specific configuration).
The first icon on the left is airplane mode. White means on, grey means off. Airplane mode controls the cellular signal (SIM card). The second icon from the left is wifi. The third icon is Bluetooth.
Configure the device as per the top example when your home wifi network is available. Leaving the cellular radio off prevents the device from connecting to the cellular network if your wifi network drops out. Cellular data is typically expensive compared to wifi data.
Using the middle configuration will allow the device to use either cellular or wifi. Wifi is preferred and cellular is used when wifi is unavailable. You can leave the device like this indefinitely if you don’t want to manually change the radio settings.
The bottom configuration will use cellular signal only. This is the best configuration when traveling. Leaving wifi explicitly turned off prevents the device from being promiscuous with randomly encountered untrustworthy wifi networks it meets along its journey.