Lilith is a White-Eyed Demon (with white pupils and irises) who was at one point human. Most demons know her as the first demon to ever be created. Lucifer corrupted her soul as a an act against God after he was expelled from heaven. She was a leader of many demons before her death. Her death was the last of The 66 Seals.Being the first demon created, Lilith is far more powerful than most demons. She has superhuman strength and superhuman durability. She is telekinetic which allows her to influence objects or people with her mind. She can emanate a demonic white blast of energy from her palm that can decimate structures, destroy supernatural creatures along with humans. Lilith can force a demon from its human host and reclaim its body by re-possessing it. She can command demons and she can bear contracts of deals made and make deals herself. She can also control Hell-hounds. It is implied that Ruby's knife can at least harm her. She can be killed by Sam (only when he has consumed vast amounts of demon blood).
Loki: A regular at Valhalla, home of the Norse gods, Loki was not a god but a particularly mischievous Trickster. Blood-brother to Odin, King of the Gods, Loki usually escaped serious retribution from his wildly cunning exploits. Often, however, the gods required he right his vicious pranks, including his theft of Idunn's Golden Apples of Immortality. Like his West African counterpart Anansi (see above), Loki possessed the ability to assume the form of various animals. It is later revealed that Loki is actually the Archangel Gabriel, therefore he has the same strengths and weaknesses as Gabriel does.
Lucifer: God created the Archangels; Michael, Lucifer, Raphael, and Gabriel. Lucifer formed an especially strong bond with his brother Michael. After the creation of mankind, Lucifer began to argue with his brothers and with God over these new, imperfect beings. The arguments became so heated and bitter that Gabriel chose to leave Heaven rather than watch the fighting continue. According to Lucifer, God commanded his Angels to venerate Man, but Lucifer refused, and was cast out of Heaven. As an act of defiance, Lucifer twisted a human being's soul to create the first demon, Lilith. He was then confined against his will in a Cage by God. The Cage could only open if sixty-six of the six hundred seals on the Cage broke. During his confinement, demons multiplied, and some worshiped him as their creator. In 1972, Azazel possesses a priest at St. Mary's Convent, and killed eight nuns in the chapel. This sacrifice enables him to speak to the imprisoned Lucifer, whose cage opens beneath the convent. Lucifer tells Azazel that Lilith is needed to break seals that hold him captive in the Cage, and that Azazel needs to find a very special child. Lucifer is freed when the last seal is broken inadvertently by Sam when he kills Lilith. While on Earth, Lucifer, with the help of the Four Horsemen of the Apocalypse, causes chaos and natural disasters. He intends to destroy humanity and to kill Michael. Essential to Lucifer's plan is the possession of his destined vessel - Sam Winchester. He is able to teleport himself and others and can overpower primordial beings like pagan gods and all lower-classed angels. He had the power to make lower angels explode with a snap of his fingers. He can manipulate weather. His presence on earth is signaled by massive tsunamis and tornadoes. He can perceive the true forms of the Four Horsemen of the Apocalypse, as he demonstrated when he saw Death's true form. He is able to freeze objects. He freezes a window by breathing on it and remains unaffected by time. He can bind entities to his command by conducting a ritual, as he demonstrated with Death. He claims that he would be able to bring Sam back to life if Sam killed himself to stop himself becoming Lucifer's Vessel. His weaknesses include "The Colt", which can damage, but not kill, him. An Archangel Blade can kill Lucifer and so can Michael. If he has the weaknesses of other angels, he can also be confined by a circle of holy oil, and kept out by Enochian sigils.
Osiris is an Egyptian god. He selects people and then confronts them with things that they feel guilty about. He can see into a person's heart, and if they carry more than a feather's weight of guilt, he sentences them to death, in a manner reflecting their guilt. He commands ghosts or spectres to carry out the sentence. He has the ability to command the spirits of the dead, causing them to become Vengeful Spirits and also has the ability to transport people. Being stabbed with a ram's horn disables him for a couple of hundred years.
Ruby is a black-eyed demon who was integral to Lucifer's plan to escape from his cage and start the Apocalypse. She befriends Sam and gets him to trust her while in reality is steering him toward the goal of freeing Lucifer. Has the same strengths and weaknesses as other demons of her kind. To see the list click here
A specter is a type of ghost that possesses someone holding a grudge over a betrayal whether it be small or not or regardless of how long ago it was. They are created, when a person dies, feeling greatly betrayed by the person who causes their death. A Specter can possess anyone in contact with the object the specter is tied to. It imbues its host with increased strength. When possessing a person, it greatly enhances their durability, one described being punched in the face as "tickling". A Specter can identify humans with suppressed anger and a severe sense of betrayal and would even prefer possessing such hosts. The Specter increases the host's volatility and prevents suppression of their rage. They secrete a green ectoplasm rather than black. When possessing someone, they can't be filmed, as their head just appears as a blip. As with any type of ghost, they can be put to rest by salting and burning their bones and/or the object they are attached to.
(1) Supernatural Wiki