[I have a book of Introduction to Angels at Amazon.com/author/billhaberman]
What is an Angel?
In ancient times the word angel simply meant a messenger. Alexander the great called his ambassadors and emissaries angels.
The ancient ones divided the messengers into three classes, heavenly messengers, earthly messengers, and chthonic messengers. [Chthonic has to do with the underworld, hades, the realm of the dead.] But the basic sense of the word was one who carried a message.
These three classes are also seen in the Christian Bible. There was the angel, a heavenly angel, that stood in the way of Balaam as he went to curse Israel.
There were the three men, earthly angels, that came to Abraham.
They brought the message of the coming birth of Isaac. There was the appearance of Samuel after his death.
He came with the message to King Saul concerning his coming death. Having, by the witch, been brought back from the realm of the dead, he was a chthonic angel, brought back from hades if you will.