We've all experienced a feeling of dissociation before, but typically under another name: daydreaming. We temporarily take leave of our senses and our surroundings and imagine ourselves to be somebody entirely different, who's leading an entirely different life. Now consider a less pleasant form of dissociation: a person drinking alcohol until reduced to a state of falling-down confusion. The person's actions, thoughts and words are seemingly not his or her own, and the events that transpired while in that state aren't remembered the next day.
People with dissociative identity disorder (DID) share some traits with both the daydreamer and those who drink until they black out. For them, the "daydreams" and surreal sense of place and time may last for weeks or months. They experience lapses of time, a loss of identity, severe memory issues and an adoption of new personalities that have their own unique traits, histories and perceptions. The person essentially becomes a helpless and sometimes unaware passenger in his or her own body.
For reasons not fully understood (though trauma plays a significant role), people with DID form alternate personalities (called alters) that take command of their body. The behavior, speech and thoughts of the alter are often radically different than those of the host, or original, personality. Instead of knowing who they are, people with DID find that identity has suddenly become a very slippery concept.
DID isn't the only disorder that disrupts memory, the senses and identity -- dissociative amnesia, dissociative fugue and depersonalization disorder fall under this same category. DID is, however, the most severe -- and the most likely to result in multiple personalities.