Radar Detectors are, as the name implies, devices designed to
sense the
electromagnetic waves emitted by
RADARs.
Radar Detectors are available to the consumer market in the form of a small device than can be hooked to your windshield and will hopefully warn of when a cop is standing around with a RADAR gun, ready to give you a speeding ticket.
Most modern detectors are able to monitor multiple RADAR frequencies used by the law enforcement agencies. The most common ones being X-Band, K-Band and Ka-Band. Currently, most models also detect LIDARs, but they only offer limited protection against those.
Radar detectors can save you from a speeding ticket, but they're no match for VASCAR, a very simple and old technology that is being re-implanted in some countries (USA included).