Vessel

A vessel is a human that an angel possesses in order to manifest on Earth. The term vessel was first used by Castiel. Vessels are the safest and most conventional way for angels to interact with humans on Earth, as their true voices and visages can kill. Vessels also appear necessary for an angel to use the full range of his abilities on Earth. Zachariah explains that even Lucifer needs a vessel outside the cage.

True Vessels
The term "true vessel" is used several times to described someone preordained to host an angel. It is particularly applied to the vessels of Michael.

Archangel Vessels

Dean Winchester was preordained to be Michael's vessel, being described as "Michael's sword". Zachariah tells Dean that Michael needs him as a vessel so that Michael can lead the The Host of Heaven in the final battle against Lucifer.

Sam Winchester was preordained to be Lucifer's vessel, who's self-righteous attitude is said to mirror Lucifer's. Sam was present in the Chapel not just to kill Lilith, but also so that following Lucifer's release he could become his vessel. According to Castiel, consuming demon blood strengthens the person's body, making it possible for him or her to host Lucifer.

Donnie Finnerman was chosen by Raphael to be his vessel. Donnie was a mechanic in Maine prior to his consent to Raphael. Balthazar destroyed Raphael's vessel using Lot's Salt.

Angel Vessels

Jimmy Novak was chosen by Castiel to be his vessel. While he initially consented, Jimmy is keen to resume his life with his family after Castiel is forcibly summoned back to Heaven. He does resume his former angelic occupation as Castiel's vessel. Unlike Dean, Jimmy can hear Castiel's true voice and perceive his true visage unharmed.

Claire Novak is Jimmy's daughter and is briefly a vessel for Castiel. After Jimmy is shot by a demon possessing his wife, Castiel possesses his daughter Claire. Jimmy, near death, begs to be allowed to take her place. Castiel agrees. A young man named Alfie was chosen by Samandiriel to be his vessel, though he may or may not be his true vessel as he refers to him as the nearest vessel when he needed one.