Their true visage can cause blindness in humans, although there are a few special individuals who are able to see their true form.
They can possess people, but they take the devout who have prayed for the possession.
The "essence" of an Angel is their Grace, which can be seen as a white gaseous form, similar to the black smoke of the demons.
An Angel can tear out their Grace and will fall to Earth to become human.
An Angel who falls to Earth will be reborn as a baby.
Some Angels fall to Earth and can become Demons.
Only four Angels have ever seen the face of God.
Angels are immune to Ruby's Knife.
They can be "dispatched" with a blood symbol, although this doesn't destroy them.
Anna's Grace as stored by Uriel.
Anna receiving her Grace back.
Types of Angels
There are different levels of Angels featured on the show, as Anna was described as being the "boss" of Castiel and Uriel. Anna describes herself as being stationed on Earth, watching humans. Uriel is described as a "specialist" when it comes to cleansing a town. These indicate that Angels can have different roles, but nothing more is known yet.
Angels In Mythology
Very little information can be found about Castiel, other than he is an Angel who presides over Thursdays.
Traditionally, Uriel is one of the Archangels. He is often represented as an Angel of Repentance and stands at the Gates of Eden with a fiery sword.