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 its intention through and through. 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 Defence Against the Dark Arts is such an important class for students at Hogwarts. Some other schools have a reputation for teaching the Dark Arts, not simply Defence Against the Dark Arts; Durmstrang is such a school.