Vessels

Vessels are individuals that have been, or are, occupied by a supernatural entity, whether it be voluntarily or involuntarily.

Characteristics
Angels use vessels as a means to physically interact on Earth. Angels require a vessel to manifest safely outside of heaven. the physiques of angels are too intense to behold and can be catastrophically damageing to most people, with the exception of the Angels own vessels.

Process
It is a law that angels use vessels to interact, and no angel can disobey it, and thus 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 form 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 to gain control of their body. 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. Angels who have manifested within a vessel have superhuman abilities that easily surpass the power of a demon. It is unkown whether angels supress their hosts like demons, but Jimmy described having Castiel inside him like being "strapped to a comet".

Archangels (with the exception of Michael and Lucifer) however are much more powerful than regular angels and as a result their presence in a human "hollows them out" leaving a vacant empty shell once they leave the human with recovery unknown.

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.

Angelic Occupants

 * Dean Winchester - was preordained from the birth of humanity to host Michael.
 * John Winchester - was chosen by Michael to serve as an alternate vessel so that he could save Mary Winchester (to ensure Sam and Dean were born) and converse with Dean.
 * Adam Milligan - was chosen by Michael as an alternate vessel.
 * Sam Winchester - was preordained from the birth of humanity to host Lucifer.
 * Nick - was chosen by Lucifer to serve as his vessel until Sam agreed to let him in.
 * Donnie Finnerman - was chosen by Raphael to serve as a vessel.
 * Jimmy Novak - was chosen by Castiel to serve as a vessel.
 * Claire Novak - served as a vessel for Castiel temporarily.

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.
 * In the latter part of The Bible, in Matthew for instance, multitudes of people are possessed by demons in order to challenge Jesus. However, by his divine power, Jesus was able to exorcize the demons from them.
 * Vessels holding demons are not responsible for their actions while possessed; as it is not their fault they have been possessed in the first place. However, vessels holding angels are responsible for their actions since they agreed to possession. It is unknown if any other types of beings need permission to gain a vessel, such as the Leviathans, but this is unlikely, as Leviathans are the first creatures to have been created and thus would see humans as so inferior it is not worth asking their permission.
 * Azazel speaks of his temporary vessel, John as being a meatsuit. Other demons have also used this phrase to describe their human host.
 * In popular culture, including films such as The Evil Dead and The Exorcist and even in popular television series such as Doctor Who and Charmed, vessels holding divine and infernal beings physically change when possessed, e.g. they gain markings on their skin or their face contorts and gains bulges, often making them unrecognizable. However, Supernatural stays very close to the Biblical account of possession, since the vessels in the show do not physically change upon possession - The only main difference is their eyes, which change colour subtly depending on which type of demon is possessing them. Also, another sign of possession is their lack of emotion, and their behaviour in general.