"The term remote sensing refers to methods that employ electromagnetic energy, such as light, heat and radio waves, as the means of detecting and measuring target characteristics."

Sabins, Floyd F., 1996

Remote sensing of the Earth's surface - also known as Earth Observation - involves creating images using spaceborne or airborne sensors sensitive to particular parts of the electromagnetic spectrum (Fig.1).

The Electromagnetic Spectrum
Figure 1: The Electromagnetic Spectrum.

As each object on the Earth's surface has a particular spectral signature (Fig. 2) and the whole electromagnetic spectrum is wider than the "visible" wavelengths seen by humans, it is possible to produce images containing much more information than can be deduced with the naked eye.

Generalised reflectance spectra of some earth surface materials
Figure2: Generalised reflectance spectra of some earth surface materials.

Remote sensing data is usually captured by sensors aboard satellites orbiting the Earth, which send back their data digitally via ground receiving stations. The digital data must then be processed into images before it can be post-processed for analysis and interpretation.

For more information see: http://imagers.gsfc.nasa.gov/teachersite/index.html.