Everyone dissociates; anytime you aren't aware of what's going on around you ("lost" in a good book, for example, or drive to work and don't "remember" the trip) you are dissociating. When you are dissociating you aren't aware, so you can't know you are doing it then :-) Did you mother ever call you to come do some chore (mine was always to fold clothes out of the dryer) and had to call you several times because you didn't hear her? Anytime you are day dreaming, you are dissociating.

People with a dissociation disorder might dissociate for hours, days, etc. If they have dissociation identity disorder, instead of just not being "there" another personality might emerge and run their life so when the former personality comes "back" they won't know what has happened/taken place in the meantime. Someone might tell them they said or did something and they have no recollection. When the rest of us dissociate, we don't go or do anything we can't figure out we did (getting to work but not remembering the route or what you were thinking, you still know you probably just drove straight to work, your usual route and that it is your usual time for arriving, etc.).