The document discusses cross-platform mobile application development. It provides information on mobile operating systems like Android, iOS, Windows Phone OS, Symbian OS, and others. It then discusses cross-platform applications which can be built using a single codebase that runs on multiple platforms. There are two main types: native cross-platform apps which are coded for each platform, and hybrid apps which are built using HTML, CSS, and JavaScript and then wrapped in a native container. Popular frameworks for developing hybrid apps include PhoneGap, React Native, and Flutter. The document also provides details on how hybrid mobile apps work, comparing characteristics of native and hybrid apps, and discussing the PhoneGap framework in more depth.