The Dark Arts differ from other forms of magic in the intent of the wizard using it. Most magic is relatively neutral--it can be used for bad or good. Some magic, however, is Evil in it's intention through and though. Spells of this kind are often called curses. Curses are spells that are often intended to cause harm to another person. This intention to do harm places that spell into the realm of the Dark Arts. However, simply casting a Curse spell doesn't mean that a person is using the Dark Arts. Ultimately, the deep, true intention of the caster is what makes the difference.

This is a difficult distinction to make in many cases. However, understanding the difference between acceptable and Dark magic is of key importance for witches and wizards in training, which is why Defense Against the Dark Arts is such an important class for students at Hogwarts. Other schools have a reputation for teaching the Dark Arts, not simply Defense Against the Dark Arts; Durmstrang is such a school.

Dark Magic spells
Typical Dark Magic spells are called Curses. There are offensive spells that shoot out of the wand like a gun; curse energy causes physical damage to things it hits besides the magical effect. Hexes and jinxes are lesser spells that adversely affect the target. As mentioned above, these spells are not necessarily Dark Magic. The intention of the caster is what actually makes the difference.

Dark Magic is more than simply curses, however. Magic which involves tamping with the free will of another person (e.g. The Imperius Curse), or which kills another person (e.g. Avada Kedavra) would be considered Dark Magic indeed. Voldemort spent years in magical research into ways of becoming immortal. This pursuit is also an example of Dark Magic, presumably because it tampers with the natural order of things.

Dumbledore referes to Petrification as "Dark Magic of the most advanced kind." (CS9)

Dark Magic items

Dark Creatures
Dark creatures differ from other animals in several important ways:

• they are magical in nature
• they often do not have a complete life cycle the way a typical animal would but exist simply as a physical extension of an evil intention
• they seek actively to harm, often for no other reason than that's what it does (i.e. not for reasons of sustaining it's own life)

Dark Creatures, on the other hand, aren't animals. They don't have life cycles in the same sense that normal animals have. They attack for the sake of hurting someone, not simply to eat. The red caps are a great example. They bludgeon unususpecting travellers so that they can use their blood to stain their cloth caps red. It doesn't give them food, it doesn't allow them to reproduce, they do it purely out of nastiness. One way of thinking about it is to say that they are a physical embodiment of an evil, harmful intent. And if you think about the essence of magic being intent, Dark Creatures are the physical embodiment of Dark Magic.

The lines do get blurry, surely. But even werewolves fit this description. Most of the time they aren't Dark Creatures, but when they are, they attack people with the intention of passing the disease on to others or kill them.

As for the pixies, they AREN'T dark creatures. That's why Seamus snickered when Lockhart showed up with them. They're simply Magical Creatures with a rude attitude.

some Dark Creatures:
grindylows
kappas
red caps
vampires (considered part-human)
werewolves (although human part of the time)
boggarts
dementors
banshees
hinkypunks
Pogrebin