Some Hardware Terminology
A quick review of some terms and concepts are useful before proceeding to a discussion of the different remote sensing devices currently being used (or for which useful archival data exists). Not all remote sensing researchers agree on these definitions; consider them to be a guide but not canon.
A "sensor" is the entire device that gathers and detects the photons. This is usually what people think of when they discuss some remote sensing equipment. The portion of the sensor which is the photosensitive electronic chip is called the "detector". (Some may use this term even more restrictively, i.e. to indicate the tiny portion of a chip responsible for the signal being sent to a single pixel.) Usually only the hardware designers care much about the details of the detectors--as long as the detectors work, the end users (ecologists, land managers, etc.) couldn't care less about the design details. A "platform" is the hardware that the sensor sits on; for example, Landsat 7 is the satellite platform that houses a number of sensors.
"Passive sensors" are those that simply gather naturally occurring photons; a person taking a photograph using ambient light is using a passive sensor technology. "Active sensors" are those devices that actually send out a beam or pulse of energy, and then analyze the nature of the energy component that is reflected back to the sensor; examples include flash camera photography, rangefinders, and sonar-using bats. I have not yet found examples of active sensors that would be suitable for remote sensing for the conservation practitioner, although LiDAR (see the related technologies section of this web site) may have some applications.
Sensors are also classified according to the altitude at which they are operated. Ground-based sensors are of course used on the ground. Meanwhile, "low-altitude" sensors are flown on aircraft a few kilometers (or less) above the ground. "High-altitude" sensors are flown on modified U2 aircraft at altitudes of 15-20 km. (Some sensors can be flown on both low-altitude and high-altitude aircraft.) Satellites are, of course, devices in orbit around the Earth at much greater distances (approximately 750 km for Landsat 7).