There's a decent general description of PID principles on
Control Technology Corp's site which discusses the technique, bode plots, etc. in broad terms.
This, and articles like PID without a Phd are good places to learn why it's needed and enough to play around with them, but like Ken said,
"its something to look forward to in college" if you really want to understand and apply them scientifically (as opposed to going by the seat of the pants).