There are two types of speed camera detectors - passive detectors and active detectors.
A passive speed detector unit works on GPS and recognises the location of sites where the location of speed cameras has been published. This type of system relies on a database of speed camera locations which is uploaded to the gps device. Updates are usually available to download from the internet for an annual subscription
Active camera detectors detect the actual beams from a camera as the vehicle approaches. Active speed camera detectors are illegal in many European countries and parts of the United States. Amongst the most well known and recommended speed camera detector makes are Road Angel, Inforad and Snooper.