[jQuery] Transparencys and Overlays

[jQuery] Transparencys and Overlays


I'm having trouble understanding how various jQuery plugins show
transparency and overlays over certain elements (or the whole page).
I think CSS provides the ability to do this, but I'm not sure.
What is the basic CSS to:
1) Put a gray, semi-transparent sheet over the whole page?
2) To do the same over just one element?
3) And to raise one element over that transparency?
Much appreciated.