PCGS: http://www.pcgs.com/grades.html (Scroll to the bottom of the page and click on the more for detailed explanations)
NGC: http://www.ngccoin.com/coin-grading/coin-details/improper-cleaning.aspx (Additional information is available by clicking on the grading problem listed to the left of the explanations)
The “Improperly Cleaned” explanation is always a favorite of collectors. Recently, a fellow collector that I met at the Ocala (Florida) Coin Club meeting told me he is still waiting to see the NGC slab that says “Properly Cleaned”. Amen!