Laser Only Training
The most basic system is a sound activated laser which you attach to the end of your weapon. The laser emits a brief beam in response to the sound of the hammer falling when the weapon is dry-fired. The disadvantage of this system is there's no real accurate way to tell where you're hitting.
Interactive Target System
The laser setup is exactly the same as with the laser only training; however, this system is equipped with the interactive target. This electronic target interacts with the laser and monitors where the beams strike the target and how often.
Virtual Target System
This system from iMarksman provides realistic training scenarios. Targets are computer generated on a LCD television or projected against a wall and a sensor camera monitors the laser beams hits and transposes them onto the target.