Alternating CurrentAdapter; when plugged to AC Power (usually 117VAC/60Hz in the United States) it produces 12VDC,24VAC or others. "UL Listed" and "Regulated" recommended.It is also called Power Transformer.
12V DC
Most board cameras,mini cameras and about one third of the professional cameras work with 12V DC100mA to 200mA for B/W cameras and 150mA to 300mA for color ones.
24V AC
About two thirds ofthe professional cameras work with 24VAC, 20VA to 40VA. The cameras usuallyhave screw type connections and you don't need to worry about the polarity.This power is usually supplied with AC Adapter and you need to prepare separatepower cable for the connection. Unlike 12VDC, this power can be transmitted toa long distance up to 450ft and is proper for the cameras that are to beinstalled out-door or when you don't have the power outlet near the camera.