When you're unconscious you're passed out or asleep. Anything going on in the body that we're not aware of can also be called unconscious.
Being unconscious can be very nice, especially when you're in the middle of a deep sleep. But if you're unconscious because you fainted or got hit in the head, that's not so good. Unconscious people aren't aware of what's going on, and they can't talk or do things — except maybe for sleepwalking and sleeptalking. There are also unconscious thoughts and desires: things going on in the mind that you don't know about.