IPhone 4S, How to Turn Off 3G?

So I just got my iPhone 4S and was wondering how I turn off 3G when I'm not connected to my Wi-Fi? It's not a problem at home but if the internet went down or if I left home with my phone it instantly switches to 3G and I'm not sure how to turn it off. I did find something under the "Mobile" section in settings that I can enable/disable 3G but it turns to "GPRS" instead. What is this and does is cost?

It's simple and easy. Go to settings / mobile and use the sliders to turn it on or off