Using internet on mobile phones have become very common nowadays. Every telecom offers data usage over 2G, 3G or 4G networks. But that is always limited and cost more money. Earlier in iPhone or iPad there was no option to track usage data over cellular network. But recent introduction of iOS 7 allows users to check the app data usage on cellular and 3G networks.
To check data usage Goto settings then select Cellular. In the cellular option scroll down and the app list. There will Data usage sorted by App. Each app data shows below each app. In the image above you can see that each App shows its data usage over cellular network. You can enable or disable to app to use data on cellular network. If disable the app option then that app will not use Cellular data.
So in this way you can check cellular data and also select which apps can use cellular data on iPhone/iPad running iOS 7 or later.