How does data get used on the verizon network?

I am looking to get a smartphone and im a verizon customer. I know that they are going to be coming out with tiered data plans on the seventh of july, if not it will be latter. My question is simply how does data get used? is it only used up when you stream video, when you upload or download, when you use apps, when you use the internet? Or is it all of those things? I am worried that the amount of internet i would use would cost far too much in extra money on a larger data plan or on the extra space used. Especially when considering that streaming 3g/4g radio is something im very interested in and the internet capabilities would be greatly utilized. Any information is appreciated! Thanks!