Cycles - Layers tab and export nodes - Can someone explain it to me please?

Hello
I am struggling to get my head around the difference with the render layer tab and the render nodes. Example: I have one scene with 2 layers plus a video background that I want to export as multilayer openexr files.
I have a Render layer for each layer plus a render layer of the full scene. Firstly, when I click in the render layer tab and choose render layer name, the checker board does not change, even if I try to change it. To be honest, I don’t really understand how it all works as, when I open the openexr files in nuke, I never seem to have what I want. I have tried to understand what is written in the wikihelp is rather short.
Also, the files rendered directly in the render node are ten times smaller than the equivalent files using the render tab.
Can someone please explain to in total newby language how this all works?
Thank you

as far as i know there are problems using openexr files in nuke. (layers don’t show up there, or something like this…)

if you setup render layers, first they are not rendered yet and appears as checkerboard (in their preview). You can render single render layers by hiting the camera butten bottom right (or left) of the render layer node.

(Note: you need to have compositor enabled of course for all this)