Demons are evil beings representing a thought, idea, action or item which could be thought of as evil, sinful or otherwise bad. Demons are more powerful depending on how many people think that thing is "bad" or not. Most demons used by Raven are the more evil demons.
Not all demons are inheritantly evil. Some are passive or even good. The biggest overall definition of a Demon is basically a long-term resident of Hell, with particular genetic oddities and powers. But Demons are not often played around with by their definition, more like their basic description
Demons generally embrace Darkness and/or Fire. The next most common elements are (in this order) Water/Ice, Technology/Electricity, Earth, Air and Light. 80% of Light Demons are Angels. Ice Demons generally live in the snowier regions of Hell.