Amnesia describes a loss of memory of things in the past as opposed to not being able to remember facts at the current time (i.e. short term memory loss). It is a symptom of many disorders and is usually temporary. Head trauma is the most common cause, but it can be caused by physiological trauma as well. It can be mild or severe, and can affect anything from a single event to a very long sequence of time. For example, a heart attack victim may forget everything that happened in the week before the attack, including the attack itself. In very severe cases, a patient may not be able to recall who they are.
Progressive neurological disorders such as Korsakoff's syndrome can cause serious ongoing amnesia.