Layouts in android is constructed from two objects: View and ViewGroup.

The View class is the base class for many widgets sub classes such as TextView and Button classes.

The ViewGroup is a view that conatains other views. The Viewgroup class is the base class for many layouts in android.

The UI Hierarchy is described in this picture from the android online sdk documentation

As you can see the root node is a view group which can be any layout that contains child views or even other viewgroups.

The root node appears on the activity by calling setcontentview ( method). Which in turn draws its child nodes and each child node draws its child nodes

In the following posts we are going to explore the categories of android widgets which are:
1. Text Controls

2. Button controls

3. List controls

4. Grid controls

5. Date and time controls

6. Menus

7. Dialogs

8. Adapters