Kinds of turbulence.

There are four types of turbulence: light, moderate, severe, and extreme.

Light turbulence causes a slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Sometimes pilots refer to it as light chop. It's the kind that rocks babies, and even a few overly worked flight attendants, to sleep. The seat belt sign may be on, but flight attendants are still able to conduct the food service with little to no difficulty.

Moderate turbulence is a little more intense. It causes rapid bumps or jolts without changes in aircraft altitude. Passengers will feel the strain of their seat belts. Unsecured objects in the galley may dislodge. Conducting a food service or checking for seat belt compliance is difficult.

Severe turbulence causes large or abrupt changes in altitude. The aircraft may be momentarily out of control. Passengers are forced violently against their seats. Walking is impossible. If flight attendants haven't strapped into their jump seats already, we may not be able to do so and we'll have to grab the nearest available passenger seat. If there's not one open, we'll sit on a passenger – any one will do. Make sure to hold on to us tightly. 

Extreme turbulence rarely happens, but when it does it will violently toss an aircraft about, making it practically impossible to control. Structural damage is possible.