Demons are fallen angelic beings who were invisible to the human body, consisting of cold and smelly, but that can mimic all sorts of colors and shapes illusory and causes a lot of scary sounds.
Psychologically speaking, they are beings who are dominated by excessive irritability, aggressiveness, jealousy and envy, although their wickedness is not an existential one, but acquired by falling from Divine good.
This fall has not only brought the malice, but ignorance. The demons do not see the angels, neither do they understand the nature of existence of natural and spiritual, and they know people only from the outside, but they could be very skilful in decoding the cues of the inner state of the body.
Demons fight against Christians in the world through material things; with monks in seclusions by careless or envious, monks and anchorites struggle with the intangible through thoughts.
To arouse passions, demons, who can not directly touch the mind-intelligence, act directly on the body or brain to cause this diverse shapes and images and altering light mind either activates tank images from the memory on the memories and at night in dreams, to arouse passionate faculties of the soul by hallucinations and passionate images.
With all the impressive arsenal imaginary and complex fantasy scenario that put him in the game demons, their power is limited and indirect one.
Demons can be combated and chased fighting thoughts they suggest through asceticism and mastering the art of discernment guard thoughts and mind.

Evagrios of Pontius