Remote sensing finds application in different situations and nature. For example, use of the light reflected or scattered from objects. With the advent of the laser, a perfect light source for active remote sensing became available, the light output of a laser has a high intensity in a well-collimated beam that can be propagated over large distances. Other uses include Lidar (light detection and ranging) and its applications to remote sensing in the atmosphere and oceanic research.
Optical remote sensing involves the use of light to observe distant objects and to obtain parameters or characteristics of those objects. It can be passive or active. Passive remote sensing would include seeing the light emitted by the object like a star, the sun, or even the headlights of an approaching car.
Remote sensing in the latter case enables one to determine the location of the object and whether it may be on a collision course or not. Passive remote sensing would also include seeing an object, but via the light from some other source that is scattered/reflected by it. Examples include seeing the moon and the planets via the light from the sun that is scattered by them, or just a person standing in the street and illuminated by the afternoon sun. In either case the light scattered or emitted by the object can also be analyzed to determine some properties of the object.
Other application examples would include the color of the object and how fast it is moving towards or away from you.