Visions are mental images of actual events, that are not neccessarily happening physically in front of the individual. These visions may be of the future, or incidents from the past. Other times, when an individual sees a vision, it may be happening at the same time, but in another locale away from when he is.
Precognitions may come in the form of visions, although a strong feeling or vibe may also count. Precognitions are usually forewarnings of future events, and not of the past.