Most major religions mention "Angels" or "Celestials," that usually appear to mankind as "Messengers from God." As a symbol, they represent the powers of the invisible world, and enlightenment.

According to the Old Testament (and therefore, accepted in Christianity, Judaism and Islam), there are nine choirs of angels, that are organized in 3 spheres. Their hierarchy is as follows:

The First Sphere -- angels who serve as heavenly counselors:
  1 - Seraphim.
  2 - Cherubim.
  3 - Thrones.
The Second Sphere -- angels who work as heavenly governors:
  4 - Dominions.
  5 - Virtues.
  6 - Powers.
The Third Sphere -- angels who function as heavenly messengers:
  7 - Principalities.
  8 - Archangels.
  9 - ANGELS.

"The angels that you're most familiar with are those in the last order. They are the ones who are closest to humanity, the ones most concerned with human affairs. Within the category of angels, there are many different kinds, with different functions... The ones that you know best... are the guardian angels. Because of shifts in their functions and your consciousness, it is useful to think of these celestial beings as companion angels. As you enter a time of increased light and love on the planet, they will not need to guard you, but rather will be your guides to greater and greater consciousness." (Ask Your Angels).

Many authors nowadays believe that a lot of the "angels" people encounter actually are higher dimensional entities or aliens. (Cf., e.g., the book "Angels and Aliens").
"Existing within the realms of fifth and sixth density, the angelic kingdom has interacted with Earth through various means. These include spirit-guide manifestation, visions, inspiration, channeling and telepathic communication. Energies appearing as angels are frequently from the Arcturus-realm."(PoL)