Active microwave sensors generate their own radiation in the microwave region of the electromagnetic spectrum. The emitted radiation is sent to the surface and then is reflected back and received by the sensor. SAR (Synthetic Aperture Radar), microwave scatterometer, radar altimeters are examples of active sensors.
Passive microwave sensors receive the microwave radiation emitted from the object. All objects emit very small amounts of microwave energy. Microwave radiometers and scanners are examples of passive sensors.