The Traditional Ghost of a semi-transparent figure in period costme is probably not a ghost at all but a 'recorded image' - somehow captured by the atmosphere or building and replayed in much the same way as a video tape. The 'ghost' usually repeats the same actions and does not interreact with the witness. The mechanism by which this phenomena operates is often referred to as the 'Stone Tape Theory'.
The Crisis Apparition is the image of a person who is or has experienced a severe trauma but isn't necessarily dead! Usually they appear to a relative or close friend - for instance, if one of your close relatives is involved in a near fatal car accident, their image is somehow projected and manifested right in front of you, appearing quite solid, prompting you to reach for the phone to check out their condition.
The Anniversary Ghost as it's title suggests, appears in the same place at the same time each year and is usually a recorded image.
The Poltergeist or 'noisy spirit' has been known to upturn furniture and throw objects around. Recent studies have shown that the activity is somehow linked with teenage girls (sometimes boys) going through puberty and could possibly be hormonal/emotional stress externalised in a way we do not yet understand.
Other variables include images of inanimate objects such as aircraft, trains, ships at sea and even buildings that mysteriously appear and disappear! Other ghosts are 'felt' or only heard, sometimes an aroma or partial vacuum is associated with the manifestation.