One app to watch out for is Google Maps...
data or 3G is the internet provided by your mobile provider while wifi is a free internet usually coming from home internet or shops like mcdonalds or hotels. wifi do not use data however due to limitation on wifi range, sometimes we tend not to notice we got disconnected to wifi causing the phone to connect automatically to 3G. if you have a plan, good. if not, then that's when it charges your account.
just make sure you turn oFF the cellular data from your mobile when connected to wifi so when you get disconnected to wifi atleast it will not auto connect to 3G. if you need 3G, then just turn cell data ON.
how to turn cell data off? it depends on the phone. iphone-settings-cellular/general-cell data // android-setting-more option-mobile network-cell data