Please explain Data Usage to me? - 1

I have an ATandT iPhone 4S. Every month I'm not allowed to go over my 12GB data usage plan, and if I do I get charged for it. Well, today my mom got mad at me and yelled at me over how my phone bill was about 100 last month, and that it was because I had gone over my monthly data usage plan.
Originally, my mother told me "data" was only the 4G aspect of my iPhone, but now she has told me it is everything on my phone (texting, 4G, using apps, etc.). This seems like a bunch of bologna to me because my mom, brother, and sister's phones all aren't charged this way (they have all had their iPhones for longer than I have).
So, can someone please explain to me exactly what "data" or "data usage" is so I know what I need to limit doing on my iPhone each month?

I don't know about texts as I have sprint and they count texting and data as separate things, but as for Data, it's every time your phone uses internet when it's not connected to a Wi-Fi network. I don't use IOS, but I know Android let's you view your data usage in the settings.

Data usage is all the information that you phone uses other than talk and conventional texts! In the case of an iPhone, iMessage often uses data and not your text allowance. So iMessage, any app usage, browsing, and email all have to come out of your data allowance!

If you go into the setting on your phone you'll find that it can't only tell you how much data you've used, but also what apps have used it! You should also be able to set a cap on your own usage, after which point data will shut down!

*The big killer on data is often streaming video - this can use a ton of data!

Data is any internet activity not done on a Wi-Fi connection.

Texting is not part of data, however the commonly misinterpreted I'm is part of data (i.e. Using apps like WhatsApp, iMessage, etc).

Apps, depending on what internet connection you use them from also would be considered data use. You can restrict which apps use data for iPhones though, so that could help as well. Of course streaming videos & music are the biggest consumers of data & should avoid doing that on a data connection if at all possible.

Data is anything to do with cellular connectivity. Downloading things, browsing the web, email, etc. (end of thinking capacity) all uses data. Calling, texting and using offline apps (ones that do not require internet) do not use data.