What exactly is using data and how can i keep it from using data when im not using my phone?

So i have an iphone 4 and i have used a lot of data this month(like almost 2GB) and i have barely touched my phone outside of wifi areas. I always shut down any apps that use data like youtube, funnypics, facebook, etc. but i do play some games during my breaks at work but they dont require data to play so to my knowledge those games arent using data(or are they?) i just need some help figuring out exactly what on my phone is using data all the time so i can turn it off. Is it icloud? Emails on auto update? Offline game apps(minecraft, sonic dash, the Mr. Series games?) Do all apps use data regardless? Somebody please answer me. And No im not buying a bigger data plan. 2GB is my limit but it seems as though i am hitting that easily with my iphone but the rest of my familyis usually miles away from 2GB and they all have Samsung Galaxy S4s.