Hierarchy variable vs Hierarchy node variable?

Hi Friends...

Might sound lil dumb..but..thanks to internet...I can hide behind my laptop screen.

Ok..

1) What is the diff between a Hierarchy Variable and a Hierarchy node variable? (might just mean wat it says!!! but pl explain..)

2)Which one to use when.?

If we wnat to use a query variable to dig into any level of a hierarchy...which one needs to be used..and how

I'm on 3.5....and I cant see a H node variable option..is SAP done with that?

a lil details steps to do the same is very very apprecaited..

Thanks for your time..

--Led