The official way of describing 'chop' is 'rythmic bumpiness.' It's what most lay people would describe as 'turbulence' and is the most common form of light to moderate turbulence that you would experience in day to day operation (usually in clear air). I sometimes describe it as 'cobblestones' as it's the easiest way to describe it to someone who only uses terrestrial transport.
'Moderate to severe turbulence' is something you would experience in 'say' cumuliform clouds (esp Cb) or in some cases of clear air turbulence found in the vicinity of frontal systems at altitude. Can be V bad. Sometimes you can't read the instruments because the panel shakes too much. Has been known to cause structural damage to aircraft. Hope this helps.