When speaking of the anatomy of android applications

Question

Problem: When speaking of the anatomy of Android applications:

1.) Java - (Why do we need it ? What does it do?)

2.) Res/drawable-hdpi - (Why do we need it? What does it do?)

3.) Res/layout - (Why do we need it? What does it do?)

4.) Res/values - (Why do we need it? What does it do?)