Please explain Data Usage to me?

I have an AT&T 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?

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.

I have an AT&T 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.
--- I average about 17GB a month on my Verizon grandfathered unlimited data plan lol lol lol

Originally, my mother told me "data" was only the 4G aspect of my iPhone,
---3G/3G/ and 4G CELLULAR DATA USAGE… Connected to a cell tower while using data = data usage against your monthly allowance

but now she has told me it is everything on my phone (texting, 4G, using apps, etc.).
--- NO! Survey Says… XXXXXXXXXXXXXXXX… Oh heck no… Data usage is Cellular data usage when connected to a cell tower and NOT free unlimited Wi-Fi… This covers internet, 85% of apps from Google Playstore (android) and Appstore (Iphones). Voice plan covers voice mins… Text plan covers texting… Example: you use the facebook app on your phone. Where the heck do you think the app gets your account info and comments and posts? Example: you use the youtube or pandora or iHeart or twitter or weather apps or online gaming or voxer or skype or etc on your phone. Where the heck do you think the app gets your account info and comments and posts? How do you think it transmits your info to your friends? Lol

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).
--- Data usage is dependant on 1 thing… YOU! Everyone has different data usage… The number 1 killers of data usage are online radio streaming (pandora/iheart), online video steaming (youtube, netflix, hulu), Online game playing (any game app that allows you to play with firends)

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?
--- see the above… And make sure you get a data tracker app on your phone. Make sure this apps measures ONLY cellular data and NOT Wi-Fi, like the fre att data tracker or use *DATA# on your phone… Most data tracker apps measure any kind of data on a phone (cellular + Wi-Fi) which doesn't give you a correct stat when you check… Make sure it only measures cellular data