This is sort of where it all begins. Remote sensing systems can be classified into 2 types : Active and Passive.
Passive sensors collect information without emitting any kind of signal. What this means is that it uses the electromagnetic energy that the sun emits and records the reflectance off of the earth. This includes anything in the visible spectrum (like aerial photograph), infrared spectrum(common in landsat). Additionally these systems can collect information being emitted from the earth such as thermal.
Active sensors require that the imaging device first emit a signal. This means that most active sensors can collect images night and day. Examples of common active sensors include LiDAR, RADAR, SONAR.
The other benefit of active sensors is that many of them can “see through” atmospheric conditions as they are not as sensitive to atmospheric moisture or haze.
Jensen, John R. (2006) Remote Sensing of the Environment: An Earth Resource Perspective, New Jersey, Prentice Hall.