From Android Wiki
Here is an overview of some of the most important package namespaces and classes in the Android API.
- android.content.Context is the base class for android.app.Activity and other similar classes. Here you will find important methods for accessing application package data and the application environment.
- android.view.View is the base class for all widgets. The android.view namespace defines non-widget-specific utility classes.
- android.widget is the namespace where you will find specific widget implementations and related classes.
- android.graphics is the namespace for the 2D graphics API; Canvas is the drawing context, Paint specifies settings like what colour, drawing mode, text settings etc to use, and Typeface is how you specify fonts.
- android.content.Intent is the basic IPC mechanism.
- android.os contains various useful classes to do with the UI framework: Handler for feeding tasks/messages to a Looper, and AsyncTask for running tasks on background threads that need coordination with the UI. Here you will also find Parcel and Bundle, which are different ways of serializing/deserializing data, mainly meant to be used for saving/restoring UI state.
The online docs tend only to describe things according to the latest version of Android, with information specific to older versions marked as “deprecated” or even omitted altogether. Such instances are noted in API version notes