Android Manifest.
The AndroidManifest.xml is a required file for every Android application. It is located in the root folder of the application, and describes global values for your package, including the application components (activities, services, etc) that the package exposes to the ‘outer world’, what kind of data each of our Activities and co. can handle, and how they can be launched.
An important thing to mention of this file are its so called IntentFilters. These filters describe where and when that activity can be started. When an activity (or the operating system) wants to perform an action such as open a Web page or open a contact picker screen, it creates an Intent object. This Intent-object can hold several information describing what you want to do, what data is needed to accomplish it and other bits of information. Android compares the information in an Intent object with the intent filter exposed by every application and finds the activity most appropriate to handle the data or action specified by the caller. If there it more than one application capable of handling that Intent, the user gets asked, which app he would prefer handling it.
Besides declaring your application’s Activities, Content Providers, Services, and Intent Receivers, you can also specify permissions in AndroidManifest.xml.
Java
This contains the .java source files for your project. By default, it includes an MainActivity.java source file having an Activity class that runs when your app is launched using the app.
Drawable
This is the directory for drawable objects that are designed for high-Density screen. We can put images in this folder. This images can be used in various Activities in our app.
Layout (activity_main.xml)
Layout is directory for files that define your app’s user Interface. . Defining the GUI structure in XML is highly preferable
Values
This is a directory for other various XML files that contain a collection of resources such as strings and colour definations.
Master Android Application Development 