A flight attendant (FA) was injured when the flight encountered turbulence during descent. According to the captain, he conducted a preflight briefing with the FA and advised of the possibility of light to moderate chop, mainly below 12,000 feet, during the descent into the Detroit area. During the initial descent, at approximately 20,000 feet, the aircraft encountered light turbulence with a few encounters of moderate turbulence for about a minute. The captain gave the final indication (two chimes) shortly afterward and then overheard the FA give the final public address (PA) over the intercom. The remainder of the descent was mainly smooth with a little light chop. Shortly after arriving at the gate and having completed the normal shutdown procedures, the FA came to the cockpit and indicated that she had been injured and requested that the captain report it. According to the FA, she was in the forward galley when the airplane encountered turbulence; she impacted the left side of her body on the galley counter. She indicated that she took a direct hit to her ribs and then fell backward and struck the cabin door with her back. After deplaning was complete, the flight crew gathered their belongings and exited the aircraft. As they exited, the FA showed signs of being in pain; as her left side was becoming more painful. The flight crew helped her to the next gate where she sat down and was met by the paramedics who briefly evaluated her before she was transported to the local hospital where she was diagnosed with a fractured rib.