How do you handle Unconsciousness in your D&D game?

First: is there some point in hit point damage where a person is unconscious but not dead? Is it at precisely 0 hp? Or do you do something else?

Second, how do you handle something that would knock someone out without necessarily reducing them to 0hp? what has to happen for someone to be knocked out? How do you determine the length of unconsciousness?

How do you handle Unconsciousness in your D&D game?