f-stop is the size of the aperture -- the hole in the back of the lens. Larger f-numbers imply smaller apertures. Unsurprisingly, the smaller the f-stop you want, the bigger your lens has to be to be able to gather all that light, and the more money you pay. The good thing about f-stop numbers is that they're totally standardized. If you took the same picture with 100 different lens with different magnifications but the same lighting, you'd be guaranteed to get the same exposure.

I assume there are some fine introductory books out there. I learned all the jargon through doing it rather than through reading anything in particular. The magic of any non-professional D-SLR is that they've got idiot modes, so you can gradually teach yourself what it's all about when and if you're going to be in a situation where it matters.

I only ever really had to figure out the f-stop thing when I had a manual-only Sunpak flash way back when. You had a knob you could twist to say how much power you wanted. They had a calculator wheel gizmo where you'd dial in your f-stop and/or film speed and it would tell you the distance that would be in proper exposure.

The place where most people worry about f-stop numbers is when trying to control depth of field. Smaller f-stop numbers yield less depth of field, getting that creamy out-of-focus blur of the background behind your subject. Also, longer lenses have shallower depth-of-field than shorter lenses, even at the same f-stop. This is part of the never-ending debate between cropped and "full-frame" sensors on D-SLRs.