For people lucky enough not to have these in their country yet, they are more commonly known as a Speed Cameras/Traps...
These are automated roadside units using Radar, contacts in the road, or laser to get an accurate measure of your speed then take a couple of pictures of you, or number plate recognition units such as SPECS that measure how long it takes you to travel a set distance to determine speed.
There are many types though; GATSO, TRUVELO, SPECS etc.
see
http://www.ukspeedcameras.co.uk/guide.htm for a detailled explaination about each kind of camera.
HTH
John