AO = Ambient Occlusion

Basically the shadow created by something that's nearby/touching something else. The shadow in between your fingers, for instance.
Basically, if you look at your fingers, there is a little shadow in between your fingers even in bright light, where the other fingers obscure the light.

Make sense? :)

Depth of Field (DoF) will push up your render time, as will ambience on lighting.

Don't dispair. I've been doing this for years and I still get it wrong. :) Practice, practice, practice. That's the key to it all.
You'll render a lot of crappy images when you start out, but with every render you learn more, see what things do, and one day you have an "A-HA!" moment and suddenly you look at it all a little differently, apply things differently or in a way you never did before.

We all still learn, Jack. Don't feel bad. :)

And as someone a lot wiser than me once said: The only dumb question is the one you never ask.
After all -- how can you learn if you don't ask questions?

Silke