Fel Magic

Fel Magic is a generally percieved Evil, and Destructive magic, used mainly by Warlocks, Demon Hunters, and is the soul power energy of Demons.

Fel is generally thought of as pure Evil, and users are commonly disrespected throughout the "General Community". There is some Non-Fel practitioners whom respect the practitioners of Fel, whether Evil, or Good intended Fel casters.

Demons
Demons are evil immortals who feed upon magic and life. In order to become a demon, a creature must be infused with Fel energy. Leading the demonic Burning Legion, Sargeras twisted many creatures into demons to join him as he worked toward his goal: the destruction of the universe, which he believed to be fundamentally flawed.

Warlocks bind demons to their will; these infernal denizens defend their masters with their lives or rain death upon their enemies. At first, they command only the service of imps, but as a warlock’s knowledge grows, seductive Succubi, loyal Voidwalkers, and horrific Felhunters join the dark sorcerer’s ranks to wreak havoc on anyone who stands in their master’s way.

Warlocks
Warlocks are magical practitioners that seek to understand darker, Fel-based magics, including destructive spells. While many warlocks willingly follow the Burning Legion, there are those who work against it, using their magic to fight against evil.

Demon Hunter
Demon hunters, disciples of Illidan Stormrage, uphold a dark legacy, the Illidari embrace Fel and chaotic magics—energies that have long threatened the world of Azeroth, believing them necessary to challenge the Burning Legion. Wielding the Fel-magic powers of demons they’ve slain, "Fighting Fire with Fire". They also develop demonic physical features as well, such as Horns, or Hooves.

References:
http://wowwiki.wikia.com/wiki/Fel#Demons (Small bits of information is taken from here.)