A mobile app is a software application designed to run on smartphones, tablets and other mobile devices. They are usually distributed through app stores operated by the mobile operating system owner, like Apple App Store, Google Play, Windows Phone Store. Popular mobile apps include social networking apps, games, news/weather apps. Developing mobile apps faces difficulties like different mobile models/OS, spam messages, copyright issues, testing apps on different devices, unclear payment systems. Developers can make money from mobile apps by selling them on app stores and websites, cooperating with mobile operators and portals, promoting through fan pages and forums, or selling directly to organizations. Location-based services use a device's location to provide information services and increase social networking interaction