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Remote Sensing is the science and art of obtaining information about an object, area, or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area, or phenomenon under investigation.

Within NASA Earth Science, remote sensing means studying the Earth system from above: from space (in orbit) or closer in, from our atmosphere. Without this method, studying the Earth as a whole system would not be possible.

There are many Earth observing satellites orbiting the Earth. Each satellite is designed to orbit the Earth at a specific altitude at a calculated velocity. This ensures that the satellite passes over the same spot on the Earth at a fixed time interval and acquires data with required amount of surface details. The sensors onboard the satellites are also carefully selected to be suitable for measuring particular components of the Earth system. For example, there are sensors specialized for measuring sea surface temperatures, atmospheric moisture, cloud composition, vegetation distribution, and rocks and minerals. Together these satellites provide a wealth of global data sets over time that makes it possible to study the Earth as a system.

REFERENCE:

Quoted from Lillesand and Kiefer in their book "Remote Sensing and Image Interpretation" and from Science @ NASA: Earth Science Remote Sensing