Waveplates, demystified - Simple-version ''(..scroll down the page a bit for the info / diag's..)''
[http://www.redoptronics.com/waveplate.html Red Optronics](Note: Commercial external-link- No 'endorsement' implied;  for Informational-purposes only..)]

..Gory-version.. :p
[http://en.wikipedia.org/wiki/Waveplate External Wikipedia-article on Waveplates]

..also, found this, too: 
[http://optics.byu.edu/animation/polarwav.mov Simple but informative Waveplate-principles animation w/ voice-over]