Think of application that resides within browser. And scope of that application is limited to the web browser. So it can't run offline. With some exceptions with apps like Pocket, which can run offline with saved data. To explain my definition, think of gmail app on mobile and gmail on browser, the instance remains common on device in online mode. Their functionality is limited to the online use. It can be made with any language and database, say like PHP, Ruby, Python etc. And can have any database like MongoDB, MySQL etc. It depends on how the app is made to function online.