There are many different types of lasers, but the basic principle behind them is the same. There are three principles behind laser light:
1) The light is all of one specific wavelength, based on the energy levels the electrons have been excited to to release the photons.
2) All the photons released move in sync with each other; that is, they don't bounce randomly about.
3) The light is concentrated. Just like the photons, it does not spread out, but stays close to itself.
The process by which lidar measures your speed is also slightly different. Where radar measures the Doppler Effect, lidar instead takes many measurements of your position. It fires out a burst of laser light at a vehicle, and measures the time it takes for the light to bounce off of your car and come back. Divide this number by 2, and you have the time it takes for the light to travel to your car, and, using the speed of light, you can then get how far away the car is. The lidar then takes many, many samples of the position of your car over a very small amount of time (think less than a second), and from there can look at how much your position has changed in that amount of time and can determine how fast your car is traveling.
http://www.telegraph.co.uk/motoring/9543633/Why-are-male-drivers-always-boy-racers-at-heart.html
Very fast.
Wave Physics
Radar, Part 1
Radar, Part 2
Lidar
Bibliography
Tl;dr