Thats because there isn't a difference between the two.

Daydreaming is thinking about other things then what you are actually physically doing - for example sitting in class and instead of mentally listening to the teacher you are thinking about that upcoming weekend party.

Dissociating is - thinking about other things instead of whay you are doing.

Normal acts of dissociating - physically driving a car down a street that you have been on before. Instead of thinking about the mechanics (push the break here, turn flashers on there, speed up, turn right turn left, mentally you are thinking boy I had fun last night, got to remember milk at the store and so on.

Every human being has and uses these normal dissociation skills.

How much a person dissocaites (daydreams is what makes it a disorder or not.

Dissociation happens on a scale of 1-10. 1 -5 being normal acts and 6-10 the various dissociative disorders - dissociative fugue, depersonalization and so on.

Dissociative Identity Disorder - where the person has dissocaited to the point where their memories have been separated and stored at the unconscious (unaware) level and are acting out those memorys without realizing the memory content and what they are doing while acting out those memories is 10 on the dissocaitive scale.