Multiple Personalities Support Group
Dissociative identity disorder is a diagnosis described as the existence in an individual of two or more distinct identities or personalities, each with its own pattern of perceiving and interacting with the environment. At least two of these personalities are considered to routinely take control of the individual's behavior, and there is also some associated memory loss,...
depersonalization is the feeling of being outside of or watching yourself. the sense of watching yourself as if you're in a movie, for example. that can be part of did, or ddnos, or it can be another disorder.
no one can self diagnose, so even if you do find a good definition of did, or ddnos (dissociative disorder not otherwise specified) you're going to need to be assessed by a professional to know for certain. we're not able to be objective enough to know if we meet the criteria, even when we have a medical issue we often miss or evaluate things incorrectly. we say in the medical profession, 'the doctor who treats him/herself has a fool for a patient.'
there are other disorders that have dissociation and depersonalization as symptoms, borderline personality disorder is one of them. there can also be some medical problems that can cause those kinds of symptoms, i know because i recently discovered that i have one - much to my amazement. i never would have suspected that some of my psychological symptoms were caused by a birth defect but it seems likely.
if you look in the dsm iv you'll find the criteria for every mental illness listed. it's being revised however so you might want to also look at the dsm v site as well.