An Android app is a software application running on the Android platform. Because the Android platform is built for mobile devices, a typical Android app is designed for a smartphone or a tablet PC running on the Android OS.
About ios app
iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that presently powers many of the company's mobile devices, including the iPhone, iPad, and iPod touch.
About windows app
Windows Store is an app store for Microsoft Windows, starting with Windows 8 and Windows Server 2012. It is the primary means of distributing Universal Windows Platform apps.