Mobile Developer Question:

src:
This contains the .java source files for your project.

gen:
contains the .R file, a compiler-generated file that references all the resources found in your project. You should not modify this file.

bin:
contains the Android package files .apk built by the ADT during the build process and everything else needed to run an Android application.

res/drawable-hdpi
This is a directory for drawable objects that are designed for high-density screens.

res/layout
This is a directory for files that define your app's user interface.

res/values
This is a directory for other various XML files that contain a collection of resources, such as strings and colors definitions.

AndroidManifest.xml
This is the manifest file which describes the fundamental characteristics of the app and defines each of its components.