This is a NJR4261J 24 GHz, K-Band, microwave, doppler radar speed sensor. It's a short range radar designed to be used in automotive safety applications for collision, , obstacle, and blind spot detection, as well as assisting in automatic cruise control functions. This particular module comes with a 7-dBi patch antenna which increases the range to approximately 15-20 m.

It operates based on the Doppler effect where a microwave signal is projected towards an object and its reflected frequency is changed based on the speed of the object. The difference between the source frequency and the reflected frequency is used to determine the speed that the object is moving at. The output from the device is only the difference of the two and is relatively low frequency.

There's a bit of theory involved, but basically, the output frequency will be in the range of 0 to 50 kHz for objects moving at everyday speeds, ie: <1000 km/hr.

This particular module has two frequency outputs. Either output can be used to determine the speed of an object. However one output is offset by 90 degrees in phase from the other. This causes one output to either lead or lag the other depending on the direction of motion. If both signals are fed into a microcontroller, the microcontroller can sense whether the object is moving towards or away from the sensor.

The output signal voltage of the module is in the range of 10-50 mV so an op-amp will be needed to boost the signal to a detectable frequency. The gain from the amplifier will determine the maximum distance that objects can be sensed at.

Here's a video I shot using the sensor:

And here's the link for the full tutorial for that video:
Tutorial: Fun with 24 GHz Radar Speed Sensors

Datasheet

2012-04-10: It seems my supplier has stopped carrying the sensor with the 10 dBi antenna and only carries the version with the 7 dBi antenna now. I can only offer the 7 dBi version at the moment.