Seraphim

Seraphs are a higher species of angel, above normal angels such as Uriel and Joshua. Zachariah is an example of a Seraph. He's worked under the command of the Archangels for at least 6 millennia.

The only known Seraphs are Castiel and Zachariah.

Physical Appearance
Their true form is very intimidating, something like thousands of feet high, with four faces, one of which is a lion, and six gigantic feathered wings. Exposing his true (altered) form, Castiel instilled tremendous terror in Crowley, causing him to shake and tremble.

Background
Zachariah was shown to be extremely condescending towards demons due to his pride and superiority, and of course the fact that they detest each other by nature. Imbued with great power, the Seraph have such strength he could smite high-tier demons. Castiel, even in a weakened state, was able to intimidate the King of Hell into fleeing when he appeared like he was about to try to smite him.

After God resurrected Castiel after Lucifer killed him he chose to upgrade his celestial status considerably, making him a Seraph. When Castiel regained his memories he was much more powerful than before, being able to smite several demons effortlessly. When he was a normal angel it took him a great deal of focus to kill demons with the touch, and he was not strong enough to kill demons like Alastair, which he now definitely can as in a weakened state he was capable of killing Crowley, the King of Hell. It's probable that Zachariah was somewhat more powerful than his Seraph brother, since he was older.

In the hierarchy of Heaven, the Seraphim, like Zachariah, are ranked higher than Cherubim and angels, but lower than Archangels. Since Michael and Lucifer are sealed away in the cage, Castiel might be one of the most powerful and highest ranking angels alive, though Naomi appears to be above him.

Unlike standard angels, Seraphim powers are imbued within them, meaning they still possess all of their powers, even if they become fallen angels, and are cut off from Heaven, as demonstrated by Castiel when he burned a monster to death with his mind, during his time in Purgatory unlike the first time he Fell and lost that power because he was cut off from Heaven. The major difference between average angels and the Seraphim is that the latter possesses several cosmic powers lower angels don't, and some they do, but at a greatly elevated level. They can bend time without effort, heal humans of any injury or illness, revive humans who has been reduced to atoms, even years later. However, being seperated from Heaven for long periods may weaken them as when Castiel returned from Purgatory, he still had all his powers, but it exhausted him to use his strongest abilities. This is apparently a temporary problem however as Castiel quickly seemed to regain his strength.

Powers and Abilities
They possess all angelic powers but somewhat more powerful. They possess these powers, but are not limited to them, as they may not demonstrated yet:
 * Biokinesis - Seraphs manipulate/influence the human body, like can inflict pain down to the molecular level, including internal bleeding, removal of lungs, and hemoragies.
 * Teleportation - Seraphs, as Angels, can move from to another, instantly; in the blink of an eye.
 * Telepathy- They can easily read the minds of humans.
 * Advanced Pyrokinesis - Seraphs can generate and manipulate fire.
 * Superhuman Stamina - Seraphs never tire and they do not require food, or sleep to function normally.
 * Enhanced Superhuman Strength - Seraphs are extremely durable and strong. They can overpower and kill any demon and monster (except for Leviathan). They can also overpower other angels but are inferior to archangels. Seraphs are also strong enough to fight two Leviathans for at least a short time when other angels are easily overpowered by them. Castiel lifted an object that weighed a ton, without effort.
 * Advanced Healing - They can heal others from severe wounds and diseases and even monster infections with ease. Castiel was able to regrow a cut-off finger on a human.
 * Resurrection - They can revive the dead as one of them brought Adam back to life and another brought Bobby Singer back.
 * Invisibilty - Seraphs can become invisible to humans.
 * Immortality - Seraphs like other angels don't age or die by natural causes. However, as is the case of most celestials they can be killed, although it's very difficult.
 * Angelic Possession - Like all angels, Seraphs need a human to possess, but because it's angelic possession this needs to be with the human's consent.
 * Advanced Killing Touch - This ability was shown by Castiel when he kills Lenore and a monster in Purgatory as well as several demons. When Castiel battled two Leviathan in Purgatory he put his palm on one of the the Leviathan's forehead, but it quickly pushed him away. It's unknown if Seraphim can hurt Leviathan this way. Castiel also appeared to be able to kill the King of Hell Crowley with this power, even in a weakened state as he fled when Castiel appeared about to use it on him.
 * Regeneration - If their vessel is in some way injured, they can regenerate very quickly, though wounds caused by angel weapons take longer. They are immune to Earthly diseases.
 * Advanced Chronokinesis - Seraphs can travel through time, with greater ease than lower angels, and they are also able to send others either forward or backwards through time, with the same ease. They can send others through time without having to travel with them as well.
 * Reality Warping - Seraphs posses low level reality warping abilities, Castiel was able to assemble a game by simply shaking the box. Zachariah was able to undo all the damage Dean did to the Green Room, in a blink. 
 * Mental Manipulation - Seraphs can manipulate, erase or restore a person's perceptions and memories of events. Zachariah was able to block Dean and Sam's true memories and implant false ones. He was later able to restore their true memories. Castiel was able to erase Lisa and Ben's memories of Dean and later show Dean the true version of what happened when he escaped Purgatory. He was also able to transfer Sam's madness into his own mind. They can even enter a person's mind and bring others inside that mental world, but require physical contact with both people to do so.
 * Soul Reading/Channeling - One searph has shown the ability to read a person's soul to check the marker another angel had left there, see if there is a soul there and check the state of the soul. He has also displayed the ability to channel power from a human soul, though this had to be done very gingerly and could result in the death of the human if done wrong.
 * Power Removal - Seraphs are capable of removing powers from empowered humans, however, it can leave them mentally damaged to do so and is very painful to the individual losing the power.

Weaknesses

 * Angel blade - As demonstrated when Dean stabs and kills Zachariah.
 * Angel-Banishing Sigil - When activated, such a mark can temporarily banish Seraphim.
 * Archangels - As angelic beings found on the highest echelon of the heavenly host, archangels can easily overpower Seraphs. Raphael, who was much weaker than Michael, certainly enjoyed and took his time beating the Seraph Castiel to a bloody pulp, effortlessly. When Castiel was so beat up he was immobilized, Raphael was about to kill him when Balthazar intervened. Zachariah believed that when Michael came to give him a second chance, he was going to kill him and didn't even try to fight back.
 * Leviathan - Being older than angels in general, Leviathans are able to overpower Seraphim, though they are capable of better holding their own against the Old Ones.
 * Common Angelic Weaknesses - They are vulnerable to Holy Oil, Heaven's weapons, etc.

In Lore
In lore and Christianity, Seraphs are a type of Angel that are known for continually worshiping and praising God, which they do day and night. Their form consists of six ginormous feathered wings, two covering their face, and two covering their feet, and four unique faces.