Data is you phone connection to internet. Wifi is your phone connected to a different device that allows you access to their internet.
So if you phones data service is "turned on" every app on the phone with unrestricted access to the data service will eat data. What ever the app wants to update, sync, etc. Some apps update your personal interests. If location is on they update personal interest in real time to your mobile location.
It's always good to know what uses data on your phone, we'll be glad to help out.
What type of apps do you use on a daily basis? Are you normally connected to Wi-Fi?