When does an iphone use data?

I want to get an iphone and i won't have unlimited data. I will always be connected to Wi-Fi at home to save my data but what I want to know is when i'm somewhere where there isn't any Wi-Fi will my phone use data to get updates and notifications. For example, if i'm at the supermarket and not on a Wi-Fi and my phone is asleep in my pocket is it still using data? Or I ski and there clearly isn't any Wi-Fi on the mountain. If my phone is on while i'm skiing is it going to be using data?

For now I have a android and will be switching to IPhone 5 within a few weeks. My mom has an android as well and we share the less amount data package that At&t provides. What we do is, turn off the data and use Wi-Fi 24/7 and home. The only time we use the data is if we're out and about, and need to look something up or use the gps etc. I'm also available to use Wi-Fi at school so there's no need to use my data. I'm almost positive you can go into your settings and turn your data off and back on when needed.

If your comfortable with turning off your data through your phone which I'm guessing you are since you don't want to get charged for over usage of data. Then turn it off because when you are out and about, where there's no Wi-Fi then your phone IS going to use the data since there's no Wi-Fi. So turn it off.