Dissociative amnesia is a condition of memory disorder, in which a person can not remember important information about his/her life.
A person with this order blocks out certain information, usually associated with traumatic events. The memories still exist but are deeply buried in the mind and can not be recalled.
This is what makes dissociative amnesia different from simple amnesia.
Simple amnesia involves loss of information from memory as a result of disease or injury to the brain.