Light Turbulence: slight, erratic changes in altitude/attitude. Occupants may feel slight pressures against straps, unsecured objects may be slightly displaced.
Light Chop: slight, rapid, somewhat rhythmic bumps without appreciable changes in altitude/attitude.
Moderate Turbulence: similar to light, but greater intensity. Aircraft is always in positive control, usually causes variations in indicated airspeed. Occupants feel definate strains against straps, unsecured objects are dislodged, walking is difficult (assuming the aircraft is big enough to walk in).
Moderate Chop: similar to light chop, greater intensity, still no appreciable changes in altitude/attitude.
Sever Turbulence: large, abrupt changes in altitude/attitude, large variations in airspeed, aircraft may be momentarily out of control. Occupants are forced violently against straps, unsecured objects disappear, walking is impossible.
Extreme: Violently tossed about, practically impossible to control, may cause structural damage.