How does data get used on an iphone4 on att?

I'm thinking about getting an iPhone 4 and I have no idea how data works. Does it get used only when you download things or is it every time you do anything like browsing the internet or using apps. What kind of data does it use up? I'm planning on getting the smallest plan that is like 200mb will I be able to do anything on my phone before it eats up the data?

Anything you do on the internet unless you use wi-fi.

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