New iphone, even more concerns?

[Ray J]

So im getting an iphone finally but i just want to make sure of stuff. Every time you get on the internet and stuff, it uses data? what is data? what stuff uses data? and say im on my wifi network at home, would that count as me using data or does it only count as me using data if im not connected to a wi-fi network (like im in a park or something on the internet). And me purchasing a data plan means i can log onto the internet anywhere, right? Not just on my wi-fi at home? Iphone experts, please explain. Thanks