Devices that run the iOS operating system are Apple products (iPhones, iPads, iPods,etc.) and other smart phones use the Android operating system. Apple products feature their downloadable apps from the App Store in iTunes, and Android provides apps via Google Play. Many apps are free, others are purchased within the app, and others are available to buy.
The library purchases subscriptions to academic databases, and these databases frequently offer a free app with the database so that its content can be viewed and utilized on a mobile device. Apps that are *formatted* for mobile devices are simply the website content coded to fit the screen of your mobile device. Most library websites and databases are automatically formatted for viewing on a mobile device.