In iOS 7, you can restrict which app can make use of cellular data when connecting to the Internet. When cellular data access is switch off for an app, all cloud access will go via Wi-Fi for that app.
This settings allows fine-grain control of cellular data usage per app, and can be helpful to keep your bandwidth usage within your monthly telco data plan limit.
Go to Settings > Cellular, scroll all the way to the bottom for the section titled “Use Cellular Data For”. Locate the app, and turn of the switch to disable cellular access for the app.
The number beneath each app name is the total amount of cellular data used. Use the number as an indicator to narrow down those apps that make heavy usage of cellular data.