Curves, levels, saturation confusion

I use PS Elements 6.

What is the difference between levels (adjusting separate channels via the levels menu) and curves?

If you adjust one eg. levels do you adjust curves as well (on the same layer? on different ones?)?

And how does saturation work after you've adjusted levels/curves? Is it correct to do this? Aren't you just duplicating work already done? And if not, do you work on a separate layer?

Many thanks in advance

This should be helpful.
http://www.earthboundlight.com/phototips/photoshop-levels-versus-curves.html

When you activate an adjustment, it's applied to how the layer looks then. With layer adjustments (at least you've those in photoshop don't know about elements) it use the settings of all that is below it.

So if you have above a curve layer a level layer you see the adjustments you make in the curve layer in the level one but not vice versa.

I hope that it makes sense.