From my manual:
Light Turbulence - Cabin Service can continue, passengers can feel seatbelts, some light discomfort. Items vibrating on tray tables
Moderate Turbelence - Cabin service should be suspended, some difficulty in walking, moderate discomfort. Passengers forced into seatbelts. Items dislodged / moved from tray tables.
Severe Turbulence - Cabin service impossible, walking around cabin impossible. Unsecured items violently being thrown around the cabin, passengers thrown around in seats.
Well, something like that anyway..
Horgy