User Interface

Your app's user interface is everything that the user can see and interact with. Android provides a variety of pre-build UI components such as structured layout objects and UI controls that allow you to build the graphical user interface for your app. Android also provides other UI modules for special interfaces such as dialogs, notifications, and menus.

The user interface for each component of your app is defined using a hierarchy of View and ViewGroup objects, Each view group is an invisible container that organizes child views, while the child views may be input controls or other widgets that draw some part of the UI. This hierarchy tree can be as simple or complex as you need it to be (but simplicity is best for performance).

android app components

App Components

Android's application framework lets you create extremely rich and innovative apps using a set of reusable components. This section explains how Android apps work and how you use components to build them.Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.

for more reference please visit to http://developer.android.com/guide/components/index.html

this will show you complete way to create android apps.