Impedance - what is it and why does it do all those terrible things to you?
Dirty Tricks you might find inside some effects and amps
How It Works - a beginner's guide to understanding what electronics is about.
The best truly basic info I've found so far - Graham Knott's Electronics for Beginners
Tom Engdahl's Basic Electonics page - a massive reference page that explains components, markings, soldering and more that you've wondered about yet. Here are a few of the topics more pertinent to effects:
These sites have even more links to other sites with basic electronics info as well. Go surfin'! Next time you will be answering that beginner's question!