My wife has a 500 mb plan and she wants to save it for important use when on a trip. So she has 3G turned off in settings, general, network. When she is away from wifi and needs to use internet, she flips it back on. Her use so far this month is 3 mb.
Where does it go? One day she was watching a video (probably YouTube) when I flipped off the wifi router to reset it. Her movie kept going . . . on 3G. No warning. (Others say this could not have happened.) iPhone is very good about not doing updates on 3G but there are features like push notification (letting you know instantly when new email is available) that use internet data. iMessage uses it when available so you don't get charged for texting. If you have Siri, be aware that Siri lives in California, not on your phone. All voice to and from Siri goes over internet. A quick look at the weather costs quite a few bytes because the whole display is a graphic.