I broused through a few web sites, if you search yahoo on
PID control feedback
you will get many links
this one gives a good overall explaintion without going into the math too much:
http://www.netrino.com/Publications/Glossary/PID.html
I found others that use fequency domain equations, if you can get past that, they are very informative - ie, skip over the math you dont understand and look at the graphs and you will get some insite.