Ghost

Ghosts that appear on Supernatural are:

• Constance Welsh, aka the Woman in White in 1.01 Pilot (episode)
• Peter Sweeney, aka the drowned boy in 1.03 Dead in the Water (episode)
• Mary Worthington, aka Bloody Mary, and Jess in 1.05 Bloody Mary (episode)
• the hook man in 1.07 Hookman (episode)
• Mary Winchester in 1.09 Home (episode)
• the doctor from Rockford Asylum in 1.10 Asylum (episode)
• Cyrus Dorian's spirit possesses his truck in 1.13 Route 666 (episode)
• the little girl in 1.19 Provenance (episode)
• Dr. H.H. Holmes in 2.06 No Exit (episode)
• Claire Becker in 2.07 The Usual Suspects (episode)
• Maggie Thompson in 2.11 Playthings (episode)
• Father Thomas Gregory in 2.13 Houses of the Holy (episode)
• Molly McNamara in 2.16 Roadkill (episode)
• Ghosts of old Hollywood actors in 2.18 Hollywood Babylon (episode)
• Nurse Glockner in 2.19 Folsom Prison Blues (episode)

One common theory of Ghosts is that they have the ability to manipulate electromagnetism, hence although they cannot communicate in the normal human spectrum of hearing they can manipulate electronic equipment to communicate by EVP. Also the use of iron to repel Ghosts has another link with the electromagnetism theory, as striking a Ghost with an object made of iron earths they're electric charge disrupting them - effectively warding them off until they're able to recover.

From Wikipedia:

Ghosts or spooks are controversial phenomena which have several possible definitions:

• The spirit or soul of a person who has died, especially one which haunts a place which was of emotional significance to that person when living.
• The personality of a person after his or her own death which is not directly tied to the soul or spirit. A sort of psychic memory-imprint.
• The character or memory of some being or thing which has died or, if it was never alive, been somehow destroyed or disassembled, which nevertheless remains existent (and sometimes detectable) in a semi-corporeal form.

According to reported sightings, ghosts have taken the shape of humans, animals, and even vehicles. There are reports of ghosts (usually of deceased humans) in most cultures throughout the world. There is a great deal of dispute whether ghosts are figments of the imagination or part of objective existence. Ghosts are among the subjects studied by parapsychologists.