What are flashbacks?

Flashbacks consist of acute events where the individual experiences significant alterations in their thoughts, feelings, and/or perceptions which are reminiscent of earlier experiences. Other characteristics of flashbacks can include resurfaced memories, depersonalization or extreme detachment, altered sense of reality, re-experiencing thought-loops similar to a previous experience, visions of former experiences or traumatic events, anxiety or panic, and the like.

Flashbacks are associated with traumatic experiences and are generally considered a symptom of Post Traumatic Stress Disorder. Flashbacks are also known to be caused by non-psychoactive drug related experiences. Any type of powerful experience can potentially lead to memories and feelings from that experience intruding into one's thinking. HPPD is not necessarily associated with any traumatic event and does not usually elicit acute panic attacks, but may accompany them.

Source:
http://www.erowid.org/psychoactives/health/hppd/hppd_faq.shtml