Apart from the WebAPIs, there are certain Web Activities which allow our app to communicate with another application to perform certain tasks, such as pick, which is one of the most frequently used mozActivity. It is used to pick images from a gallery or capture fresh ones from a camera. You can write your own code to access the photo from the device storage instead of using pick activity. However, it is generally used to provide a common user experience for app-to-app interactions, and also to provide native hardware access (as in camera). Inter-app task delegation can also be performed using web activities. If you want to send an e-mail, then you would not program the functionality of e-mail using server-side languages, instead, you would use a new web activity to create a new e-mail using the default e-mail application or any third-party application that is installed in the device.
All the actions that these activities perform are chosen by the user. The user has to choose...