Figure
Our first look at the project directory map: 
Next we need to know part:
- src directory: contains all the files needed to program code App, we are here most of the time we write Java code
- gen directory: only concerned R.java file, which is automatically generated by the ADT, which defines a class R can be regarded as a id (resource number) of the dictionary containing the user interface, graphics, and other resources of the string id , and we usually use the resource file is invoked by R, and the compiler will look at this list of resources, the unused resources will not be compiled in, you can save space for the App
- assets directory: storage resources, and no longer generate resources under id R.java file, you need to access the class AssetsManager
- libs directory: to store some jar package, such as v4, v7 compatible package, or some third-party packages
- res Resource Directory: storage resources, drawable: storage of pictures resources; layout: storage interface layout files are XML files; values: description file contains parameters using the XML format, such as string.xml string, color.xml color , style.xml styles, etc.
- AndroidManifest.xml profile: control of the file system, used to tell some of the basic information contained in App Android system, such as components, resources, as well as the required permissions, as well as the minimum compatible version of the SDK, etc.