By the author of the Arduino PID library this is a great series of blog posts explaining PID functionality. He also has a lot of good tips on how to tune.
http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/
dave
http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/
dave