When you create android application the following folders are created in the package explorer in eclipse which are as follows:

src: Contains the .java source files for your project. You write the code for your application in this file. This file is available under the package name for your project.

gen —This folder contains the R.java file. It is compiler-generated file that references all the resources found in your project. You should not modify this file.

Android 4.0 library: This folder contains android.jar file, which contains all the class libraries needed for an Android application.

assets: This folder contains all the information about HTML file, text files, databases, etc.

bin: It contains the .apk file (Android Package) that is generated by the ADT during the build process. An .apk file is the application binary file. It contains everything needed to run an Android application.

res: This folder contains all the resource file that is used byandroid application. It contains subfolders as: drawable, menu, layout, and values etc