Dark Arts – Harry Potter Lexicon

The Dark Arts are many, varied, ever-changing and eternal. Fighting them is like fighting a many-headed monster, which, each time a neck is severed, sprouts a head even fiercer and cleverer than before. You are fighting that which is unfixed, mutating, indestructible.

-- Severus Snape, (HBP7)

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.

References from the canon

On Harry's first night at Hogwarts during the feast, Percy tells Harry that Professor Snape "knows an awful lot about the Dark Arts." Since this coincides with Quirrell/Voldemort making his scar burn, Harry connects Snape with the Dark Arts from that moment. What Percy meant was that Snape fancied the job of being Defence Against the Dark Arts professor (PS7).
Albus Dumbledore refers to Petrification as "Dark Magic of the most advanced kind" (CS9).
Knockturn Alley is described as a place "devoted to the Dark Arts" (HBP6).
Being a Parselmouth and speaking snake language is connected to Dark Magic but that alone didn't make Tom Riddle dark (HBP13).
Dumbledore told Harry that the best protection against seduction by the Dark Arts is the ability to love (HBP23).
According to Rita Skeeter, the "demon" Gellert Grindelwald was educated at Durmstrang, a school famous for teaching the Dark Arts. But eventually they expelled Grindelwald when he became too horrible (DH18).
In the seventh year according to Neville, students being taught by Amycus Carrow were expected to practice the Dark Arts instead of defense against them, and Crabbe and Goyle were excelling at it (DH29).