Banshees are a breed of demons with distinctive white hair and a high pitched scream. Their screams can burst glass and blood vessels, killing mortals or turning emotionally confused witches into new Banshees.
Powers of a witch are capable of vanquishing a Banshee, as seen when Piper Halliwell blew up a Banshee with her new power. When a witch is transformed into a Banshee, the only way to revert her is for her to deal with her emotional pain. However, the transformation will become permanent when she kills a person.
The Book of Shadows has an entry on Banshees, as well as a spell to track them. However, this will turn the caster into a dog, as only they are capable of hearing the cries of a Banshee other than the victim. The Book however lacks information on how to vanquish them, but Banshees are vulnerable to powers such as Molecular Combustion.