Supernatural hunter's guide

Demons

Demons are malevolent spirits created by Lucifer. They are created from corrupted human souls that have endured extensive torture in Hell by Alastair and other demons. In this process, they become corrupted, extremely evil, and also very powerful. The species as a whole functions as the series' primary antagonists and are the most recurring supernatural creatures in the series. Similar to Angels, they require a Vessel to walk the earth, though are able to roam in their smoke form.
Weakness

Banishing, Harming, Misleading or Trapping
Devil's Trap - A Devil's Trap will hold a demon in place and prevent its escape. No matter how powerful the demon is, they are bound by the Trap. The Trap will also keep demons from using their powers. Castiel's Enochian circle was able to completely bind even Alastair's abilities. It also locks the demon inside its host unless it is exorcised. For some reason, a demon inside a trap is usually unable to do anything to damage the integrity of the circle, which would allow its escape. A Devil's Trap engraved into a bullet and shot into Abaddon's head was shown to limit her movements like a normal Devil's Trap would and to bind her to the body she was possessing.
Holy water - Holy Water burns demons that come into contact with it, like acid (though it does no damage to the human host, only the Demon). It has proved to be very effective against demons, as only Azazel and Lilith have been thought to be immune to it. However, certain high-tier demons seem to develop some resilience to it as Ruby practically laughed when Sam was about use it against Alastair because it would barely slow him down and, while it caused them pain, it only affected Crowley and the Seven Deadly Sins for a very brief period of time.
Exorcism - Will pull the demon out of its host and send it to Hell. Hosts may not survive. Before Sam and Dean learned that demons could be killed by the Colt or Ruby's knife, exorcism was the only known method used by hunters, to stop demons. Reciting the exorcism backwards will force the demon back into its host. Very powerful demons are immune to exorcism (e.g. Abaddon)
Flesh Sigil - By carving a particular sigil into a demon's possessed flesh, it is possible to lock the demon within its host and disable its powers, the most important one being teleportation. However, demons with these sigils are not bound to a single place and can roam around freely if not restrained by some other means. (The Devil You Know)
Ghosts - Ghosts can pull Demons from their vessels as shown when John Winchester's spirit grabs Azazel.
Hallowed Ground - Low level demons, are unable to enter hallowed ground, however stronger ones can. It is unclear if this includes cemeteries as well as churches. (Salvation)
Hex bags - Hex bags can be used to hide people from demons as well as angels.
Holy Fire - Though not fatal to demons as it is to Angels, Holy Fire can cause demons extreme pain, as demonstrated when Castiel threw Meg into Holy Fire (Abandon All Hope...) and when Sam doused Abaddon in Holy Fire, causing her so much pain she fled both the scene and her vessel (Sacrifice). Another demon committed suicide by dousing himself in Holy Fire (Soul Survivor)
Iron - A demon can't touch iron without its skin burning or cross it if it is used to erect a barrier, much like salt. Samhain and Alastair were immune to this weakness and Astaroth had some resistance to it. In large groups, Demons are able to overcome this weakness, as the Demon army was able to break an iron Devil's Trap in order to escape.
Palo Santo - Basically a wooden, hybridized version of salt and holy water. When a demon is stabbed with it, it causes the wound to smoke and painfully pins the demon to a single spot, immobilizing them long enough to perform an exorcism. Palo Santo isn't commonly used by many hunters due to the fact that it damages the hosts the demons are possessing. (The Magnificent Seven)
Physical Damage - Though demons themselves cannot be harmed by conventional means, severe damage sustained by their vessels (such as severed limbs) will greatly reduce their effectiveness in combat. Unlike angels, most demons are unable to heal their vessels. As such, if the vessel accumulates too much damage, the demon will be forced to vacate its host and search for a new vessel. However, some demons choose to heal their host's injuries, although this is relatively rare.
Purified blood - The blood of a person who has confessed his/her sins or is blessed by a priest is shown to be able to hurt demons if injected into them. This hurts Knights of Hell more than regular demons. It can also be used in the context of the Demon Curing Ritual.
Demon Cure - It's possible to turn a demon into a human again if, on sacred ground, a human injects his own "pure" blood in a Demon. The blood must be given hourly for eight hours. In the last dose, the blood must be fresh and the donor must tell before the exorcism: "Exorcizamus te, omnis immundus spiritus, hanc animam redintegra, lustra, lustra". (Clip Show) Alternatively, the curer only need inject the purified blood and it will cure the demon. (Soul Survivor)
Salt - Demons are unable to cross a line of salt and can do very little to damage the line at all. They cannot open doors lined with salt if opening it would break the line. Salt can also be used to torture demons by making them ingest it or by injecting them with saltwater. Forcing a Demon to ingest salt will drive them out of their host. It can also be put into shotgun rounds to hurt and knock back demons. However, Azazel is immune to this weakness and Crowley was hardly affected by rock salt shots, but he still found the experience painful.
Ingestion of Salt - Under some circumstances, the ingestion of large quantities of salt can force a demon to vacate its host. (