Visions are mental images of actual events, that are not necessarily 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. Precognition usually serves as a forewarning of future events, and not of the past.
Prophets as well as psychics, and some of the Special Children are typically the only people who receive visions, however ghosts can also provide someone living with a precognition, whether said person is psychic or not. Visions can also be implanted through the use of hex bags.
Angels are also able to produce visions in people.