In contrast to the traditional X-ray machine, which detects hard and soft materials by the variation in transmission through the target, backscatter X-ray is a newer imaging system which detects the radiation which comes back from the target.
This shows things such as organic materials as bright white and metals as darker. These systems are being used to spot "terrorist" issues, as well as for anti-smuggling, non-invasive search (rightly or wrongly) and fault detection.
Resolution is high. These X-rays are able to penetrate up to 12" (30cm) of solid steel.
In theory, these machines would allow a street to be searched by a drive-by scanning with X-ray backscatter machines. In practice, high cost keeps this from happening, but with the ever present push for more homeland security these machines may soon be in use at airports, docks, and even nightclubs.