While in cruise at FL340 and experiencing occasional light chop, the flight crew was notified by ATC of pilot reports of moderate turbulence along their route of flight. The seatbelt sign was on and the captain advised the flight attendants (FA) to stow galley carts and be seated. The first officer subsequently made a PA announcement to advise passengers to remain seated with seatbelts fastened and for FAs to be seated immediately. Soon after this, the airplane encountered turbulence, categorized by the flight crew as severe, which resulted in altitude fluctuation ± 300 feet. The FAs were in the process of returning the meal service carts to the aft galley when the turbulence was encountered and reported that they, along with the service carts, were lifted off the floor about three times. One FA reported being thrown into the ceiling and back to the seats and floor. The lead FA advised the flight crew that some of the passengers and FAs had received minor injuries but none requested medical attention. The flight continued to ORD and landed uneventfully and emergency medical met the airplane at the gate as a precautionary measure. After returning home, one flight attendant was diagnosed with a fractured lumbar vertebra.