Tube Amps, Some Basics

Hi, can anyone give us newbies a primer on tubes? Assume that I know nothing, that is what my wife does. Amps in general reproduce voltage changes, right? ie the input signal to an amp varies in voltage, with constant amperage? How does a tube amp do this vs. solid state? And what is the difference between a single ended and push pull amp? thanks.....Mark
...and what amps need "adjustments" (as the tubes age, I believe)? How do you know when an adjustment is necessary and you did it correctly?
Email A'gon member "Clueless" for some material on this subject. He has written a guide to tube amp design, and FAQ about tube amps. I have reviewed this material, and found it quite informative.