Unlike downloading software via your web browser, you access and download mobile apps from your mobile device’s store (or marketplace). The app store is the distribution center that gives you access to the correct versions of each app as well as tracks your purchases. It also empowers Google, Apple and other distributors to collect the usage fees accordingly. For example, if you want to distribute a mobile app over iPhones, you have to register for the Apple Developer Program to do so. Apple controls the applications that can be distributed and they can monitor your app distribution as well.
Some apps can be downloaded free of charge because they may charge you a subscription for the service the app provides, or charge you later to upgrade to the “full” version of the app, or they advertise within the app. Other apps are extensions of your PC software that allow you to go mobile, such as your online banking apps.
Many of these apps communicate with servers in the cloud each time you load them. They communicate with servers for several possible reasons: to sync content, to grab data from the server, to upload data from the mobile device to the server, to stream video or music to the mobile device, to backup data, etc.
If you do not kill the app when you’re done working or playing in it, you should expect that it’s still running behind the scenes (even if you’ve switched to a new app) and could be using up the data on your cellular plan.
Learn how to shut down your apps:
On the iPhone: Double-click the home button to see your most recently used apps. Swipe right or left to see all open apps. Find your app to close and swipe up on the preview of it to close it down.
On the Android: Open the settings app and choose apps. Touch the running tab to see what’s open. Tap the app you want to close and tap “stop” or “force stop.”
Keep tabs on your data usage -- if you have limited data coverage on your device, you may be shocked to receive the extra data usage fee.