The example I always like to give is, you know how it's possible to drive a familiar route and one day realize that you don't really remember the trip itself? That's a form of dissociation. I took my pills (I think) the other day, and about 2 minutes later I could not remember if I'd taken my pills. I tried to go off of where the bottles were, where my water was...and hoped for the best. i couldn't remember taking them; Icouldn't remember not taking them - I simply couldn't remember the last 2 minutes at all. Also dissociation.
That's one end of the spectrum, the rather innocuous end that could definitely be described as "advanced zoning out".
When we talk about it in psych terms, it's usually a symptom associated with one or more disorders. So, as a symptom of PTSD, it is generally brought on by general stress or a specific trigger. Some people experience it rather mildly and can observe it in the form of depersonalization (feeling like you aren't in your body) or derealization (feeling like you aren't in the world around you). And as a symptom, because it's been caused by a trigger or stressor, it can be more challenging for the person to pull themselves back into full reality. This is Depersonalization/Derealization Disorder, if it's happening all by itself; or, it can be a symptom of PTSD, Acute Stress Disorder...probably others)
(Personally, I experience this as a foggy, sleepy sensation, and I'm lucky in that I don't go anywhere or do anything; a bit like a robot that's been turned off.)
Deeper into the spectrum, lost time starts to be a problem; not as short or benign as the 'lost time' I described with taking my pills or driving somewhere, or even just 'shutting down' - but longer durations where you realize that you may have performed a series of actions, appearing fine - but you can't recall those actions at all. I understand that as Dissociative Amnesia, when it's stand-alone - or, again, as a symptom set of PTSD, and other disorders.
And next to that we have DID - dissociative identity disorder - where there is actually at least one (can be more) separate personality existing within the person, distinct from themselves, who acts during those times. That other personality (the alter) will have a memory of what transpired. This is most complicated because the individual is often very high functioning, and may or may not have an awareness of their alters. I think DID is always a stand-alone disorder, and can be co-morbid with PTSD - but I could be wrong about that.
DDNOS - dissociative disorder not otherwise specified - is what's left as a category for anyone who doesn't fit neatly into the other available classifications.
I very well might be missing some details in description - but that's my personal understanding of the Dissociation spectrum, based on personal experience and incorporating the traditional DSM categories.