If you work with electronics at any skill level you need juice. [Jon] has a great, and clearly worded tutorial about with pretty much everything you need to know about those little black boxes hanging off of your outlets.
The whole thing starts off with the basics like transformers, rectification smoothing and regulation, then moves on to the different basic types, dedicating a page to linear, regulated and switching types, giving output performance charts under different situations.
Also included is a run-down of DC barrel jack structure so you get the right plug every time, wall wart type identification, a random sample comparison test, and a good selection of formulas to even keep the old hats reading along. Although you might want to set aside a little time at 9 pages and some Q/A in the comments, it might take a moment to read.