As acronyms go, LIDAR and LADAR are a rarity--near-identical twins with essentially the same meaning. They were coined to describe the same concept, using pulses of laser light instead of radio waves to measure distance. Radar itself is an acronym for RAdio Detection And Ranging, coined by the U.S. Navy in 1941, so it was logical to replace the radio part of the acronym with an optical term. However, some people replaced the radio with light to make LIDAR and others replaced it with laser to make LADAR. Both terms are still used--although Google searches put LIDAR far in the lead, with 19.8 million hits compared to a mere 503,000 for LADAR.
The earliest and simplest lidars were laser rangefinders, which used laser pulses to measure the distance to a military target or some other fixed object. Lidars also can measure speed by firing a series of pulses and calculating how fast the measured distance changes, an approach used in police laser radars because it's simpler than Doppler measurements.
More advanced lidars scan the beam across a target area to measure the distance to points across its field of view, producing a three-dimensional profile. This technique has a wide range of uses. Lidars looking down from aircraft or satellites have profiled terrestrial terrain, and the laser altimeter on the Mars Global Surveyor spacecraft similarly profiled the surface of Mars. Combining lidar profiles of terrain before and after an earthquake can reveal changes caused by the tremor. Lidars can map archeological dig sites or dinosaur trackways too large or too fragile to record in any other way.
Specialized lidars make other measurements. Differential absorption lidar can profile the abundance of water vapor in the atmosphere. Doppler lidars measure changes in the spectrum of pulses scattered by the air to determine wind speed and turbulence.