what does android.R.layout do? An overview

Im trying to get an overview of what android.R.layout does in an android project.

From the spinner tut at http://developer.android.c开发者_开发百科om/resources/samples/Spinner/index.html

 this.mAdapter = ArrayAdapter.createFromResource(this, R.array.Planets,
                android.R.layout.simple_spinner_dropdown_item);

I can see a private member

public static final int simple_spinner_dropdown_item

But what is this?

It's a list of layout resource IDs in the android package