What is it all about?
Sensu stricto, remote sensing is the observation of an object from a distance. In practice, however, the term remote sensing involves all techniques gathering information about features on the Earth's surface through instruments onboard aircrafts or satellites.
While analogue cameras used to be the only devices available, modern platforms carry essentially two types of digital sensor systems known as 'active' and 'passive'. A passive system generally consists of an array of small sensors or detectors, which record the amount of electro-magnetic radiation reflected and/or emitted from the Earth's surface.A multispectral scanner is an example of a passive system. An active system propagates its own electro-magnetic radiation, and measures the intensity of the return signal. Synthetic Aperture Radar (SAR) is an example of an active system.
The digital data acquired can be used to reconstitute an image of the Earth's surface quite similar to an aerial photograph. With each object having specific electro-magnetic characteristics, different objects can be identified. The digital data products can be viewed and manipulated on a variety of software systems.
A short introduction to Remote Sensing techniques can be found here.