According to flight crew statements, they could see a line of clouds ahead of the airplane during descent and the flight encountered turbulence soon after entering the clouds. FDR data indicated the airplane was level at 14,000 feet at about 280 knots when there was an approximately 10 second period of fluctuations in both vertical and lateral acceleration. The vertical acceleration fluctuated between about +.426 g's and +1.723 g's and lateral acceleration fluctuated from about +.156 g's to -.211 g's. According to flight attendant (FA) statements, the flight encountered three "jolts" of turbulence about 30 minutes before landing. At the time of the turbulence, one FA was assisting a passenger with a drink in the aft galley. The FA braced herself during the turbulence, however, a passenger standing by the galley fell onto the FAs left lower leg, pinning her to the floor. The FA was assisted into the last row of seats in the cabin where she remained for the rest of the flight and the passenger required assistance to return to her seat. The flight crew indicated that the seatbelt sign was selected on about 5 minutes prior to the turbulence encounter, however, no PA announcement was made. The FAs reported that the seatbelt sign was not turned on until after a FA called the flight deck after the turbulence encounter. The flight crew advised ATC of the injuries and requested paramedics meet the airplane at EWR. The FA was transported to the hospital and diagnosed with two acute fractures of the left fibula.