Vessels

Vessels
A vessel is a human that has been occupied, whether voluntarily or involuntarily, by a supernatural entity in order for that entity to be able to physically interact in our realm or to move around undetected. Both demons and angels require a vessel if they wish to interact with humans; demons espesially considering they are non corporeal without a host whilst angels need a host for similar reason but also to shield humans from being exposed to their true forms which can be overwhelming to most humans.

Demons will usually take the host by force, somtimes attacking a potential host without them being aware of it or sometimes chasing them in order to torment the victim first. Once cornered the demon will then force itself down a vessel's mouth in the form of black smoke giving the appearance that the person is choking. After that, the demon has full control over the vessel and will remain undetected thanks to there ability to use the vessel's memories in order to fool their friends and family. Howeverm demons have little or no emotion which may lead to suspicion; forcing a demon to "deal" with anyone who comes to close to discerning the truth. Normally a demon will keep the human personality suppressed or restrained, forcing the vessel to watch the atrocities they are forced to do. Some humans, however, espesially those with a strong will, can resist the demon's control and temporarily regain control of themselves in order to try and warn people of their condition.

A demon can be exorcised from a victim with the proper ritual but the process is described as very painful and traumaticm and it is not uncommon for the vessel to be killed in the prosess. Ruby's knife has been shown to kill demons but at the expense of killing the host as well; however in Season 5 it has been shown that if somone is stabbed with the knife in a non lethal area, then the demon can be killed without killing the vessel.

Angels, on the other hand, will only take a willing vessel and will not and can not force themselves into a human like demons, thanks to the laws of God. Because of this, and the fact that most humans cannot stand the sight of an angel's true formm the angels have to find willing vessels. After finding a potential host, the angel will then communicate with the human through visions in order to explain themselves and ask for the human's consent in becoming a vessel. Such as when Castiel contacted Jimmy through his household electronics and showed him vision of what was to come. However, Lucifer tormented his vessel in order to get his consent through psychological hallucinations. Angel vessels, like demons, have superhuman abilities that exceed or even surpass their demonic counterparts. It is unkown whether angels supress their hosts like demons, but Jimmy described having Castiel inside him like being "strapped to a comet".

Archangels however are much more powerfull than regular angels and has a result there presance in a human "hollows them out" leaving a vacant empty shell once they leave the human with recovery unkown.

Angels can leave the host whenever they want to or if they are called away by the higher ups, leaving the vessel with little or no memory of the time they were occupied. The process also doesn't kill the human either. Also, as angels do not need to eat, a vessel may find themselves extremely hungry after being freed from posession.

Known vessels
Jimmy Novak (Castiel's vessel)

Jimmy's daughter (Briefly)

Donnie Finnerman (Raphael's vessel)

Nick (Lucifer's current vessel)

Dean Winchester (Michael's vessel)

Sam Winchester (Lucifer's true vessel)

Bobby Singer (Briefly)

Julia Wright 

New York Teacher (A New York teacher kills exactly 66 kids)

Father Gil 

Casey

Entity's
The Angels of Heaven - Angels need a vessel to be seen. The angel needs the humans permission in order to be a vessel. Angels can talk to humans from heaven, this was proven when Castiel talks to Jimmy from heaven.

The Demons of Hell - Demons need a human host to be seen. Demons can talk to each other in Hell, this was proven when Alastair talks to Dean and convinces him to torture souls. While on earth demons need a vessel.

The Archangel Lucifer - Zachariah states that even Lucifer needs a vessel and he needs the humans permission in order for the human to be a vessel.

The Four Horsemen of the Apocalypse 

The Two Witnesses of the Apocalypse 

God's
God 

The Archangel Michael 

The Archangel Lucifer 

Loki  

The Antichrist 

The Archangel Gabriel (Demi God) - The trickster is a demi god. The height of his power is currently unknown; He can effortlessly create anything from thin air, whether it be chainsaw wielding psychopaths, scantily clad women, food, and even an exact duplicate of himself. The trickster has always been Gabriel; this was made clear in the episode "Changing Channels".

Trivia
The Bible talks about pigs being possessed by demons; only for short periods of time. However this has yet to happen in this show.