Amnesia is a general term to describe the loss of memory, most often due to a traumatic event or injury to the head. There are two types of amnesia: retrograde and anterograde. Retrograde amnesia refers to the loss of past memories and can vary from person to person.  This memory loss can range from recent events all the way up to wiping out an entire life worth of memory. Anterograde amnesia refers to the inability to form new memories.  If you've seen the movies Memento or Clean Slate, that is antereograde amnesia.  The afflicted person cannot remember new events and can often meet the same person several times in a single conversation.  The most famous case of antereograde amnesia is H.M., an epileptic patient that had his hippocampus removed to alleviate his condition.  An affect of the surgery had caused problems with the encoding, storage, and retrieval of new events but not older events.

Chronic alcoholics often suffer from memory difficulties.  This was classified into a syndrome in the 1800's when Russian scientist Korsakoff associated the alcoholism with memory loss.  Korsakoff's syndrome  causes the loss of memories from recent events while longterm memories, like name, family members, etc. usually remains intact.  Symtoms of Korsakoff's include: retrograde amnesia, anterograde amnesia, they make up stories rather than admit memory loss, may be unaware that they have lost any memory.  Another aspect of Korsakoff's syndrome is a decrease in the size of the mamiliary bodies

Alzheimer's Disease is the most common form of dementia, in which cognitive functions are progressively lost.  In early stages, often relatively selective amnesia, sometimes accompanied by some loss of previously well-established memory and perhaps poor short-term memory.  Aproximately 3% of humans develop Alzheimer's Disease in the later stages of life.  The disease affects cells in all parts of the cortex of the brain, as well as some surrounding structures, impairing a person's abilities to remember new and past events.  The person eventually loses all memory and mental functioning.  Doctors are still finding more information on the disease and searching for a cure.
 

Posthypnotic memory loss is when under hypnosis a person can be guided to forget just about anything.  The general purpose is to purge unwanted memories as part of therapy.  Hypnotic suggestions do not work for all people but for those who are prone to suggestion it can be very helpful.  There is usually a signal to break the memory loss.