As far as I know, there's 2 kind:
1. It a kind of laser projector that projects the image of a keyboard on a surface (let's say a table) and then when you type on the table it detects key press. Never tried one, but seems awful for the fingers.
2. I saw someone make a concept on kickstarter if my memory is good where he put a laser sensor on some switches to be able to detect their movement like a joystick. Instead of only detecting a on/off position, it would detect a range of positions and translate it to the computer in some way similar to a joystick. The demo showed it's use in a fps to move more slowly I think. Never saw one other than this concept.