What does data mean in an iphone data plan?

I just got an iphone 5 from verizon with the cheapest data plan. I know data means using the internet (email, facebook, youtube, netflix). I was wondering if having multiple browsers open wastes data, also if it is possible to use data while the phone is off. I know this might sound stupid but does "data" mean taking a picture, listening to music, playing games (downloaded apps), siri?

Does syncing songs from itunes library (on laptop) to my phone take up data? I'm really confused, please help.

You're only using up data when you're not connected to a Wi-Fi network and using the Internet or Internet-based apps. Don't worry too much about using up your data. If your plan includes 2 GB of data per month, it's more than enough (unless you're constantly on the internet, lol).

To check and see how much data you have left for the month, punch in the number *3282# and press "Call". You will receive a text message with the data information. And once you've used up 65% of your data for the month, you'll receive a text message notification. If you use up all your data before the month is over, you'll receive 1 GB of extra data and a fee of $10.