how to use layers in unity 3d

here is my new query:
i have used layers in many different 3D applications but here in unity i feel it works in different way.


so i just wanna learn those basics so that i could start working on the layers provided and could achieve a better result.

Any comment or suggestions are welcome.

 

regarding layers when using gui textures you can move them by altering the z axis position...example 50..would be closer 10 etcetera hope i explained myself correctly