A CRJ-900 Flight Crew declared an emergency and expedited their arrival when passengers and Flight Attendants smelled an electrical odor and/or smoke in the cabin.

Narrative

In the descent around FL180 and approximately 60 to 70 miles from the airport; my Flight Attendants chimed me. They notified me that they and several passengers could see and smell smoke in the cabin around rows seven and eight. The smell; from what the Flight Attendants told me; seemed electrical in nature. In the flight deck; we were not receiving any messages; cautions or warnings. I also did not see any tripped circuit breakers or any information directed to me that anything was wrong.Being on the cautious side I declared an emergency with Approach and asked for direct to the runway. I notified the Flight Attendants that we were heading directly to the airport and that CFR would be waiting for us on the runway and would inspect the cargo bins and the aft cargo compartment and do a thermal imaging scan to look for heat/hot spots on the aircraft. I also advised it would not be an emergency evacuation unless the smoke got worse in the descent. At this time we ran and completed the smoke in cabin check list.In the descent the Flight Attendants told me that the smell and smoke seemed to be dissipating as we were descending. I chose to stick with the emergency plan regardless. We landed and brought the aircraft to a stop on the runway; shut down engine one and told the passengers to remain seated while CFR inspected the aircraft. After CFR did a thorough inspection we taxied to the gate without incident. I do not know why or what caused the smoke/smell in the cabin. After talking to Maintenance we thought it might have been an overhead light that burned out but we were not sure.