The Dark Arts differ from other forms of magic in the intent of the wizard using it. 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.