Up to now, the focus of this book has been on activities opened directly by the user from the device's launcher. This is the most obvious case for getting your activity up and visible to the user. And, in many cases, it is the primary way the user will start using your application.
However, the Android system is based on many loosely coupled components. The things that you might accomplish in a desktop GUI via dialog boxes, child windows, and the like are mostly supposed to be independent activities. While one activity will be “special,” in that it shows up in the launcher, the other activities all need to be reached somehow.
The “somehow” is via intents.
An intent is basically a message that you pass to Android ...