The defense against the dark arts class inself seems to be cursed since no teacher has lasted for more than a year in this position.  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 and is intended to cause harm to another person.  Spells of this kind are often called curses. 
Curses are offensive spells that shoot out of the wand like a gun.  In addition to the magical damage, curse energy causes physical damage to things it hits. Hexes and jinxes are lesser spells that adversely affect the target.  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 is more than simply curses, however. Magic which involves tampering with the free will of another person  The Imperius Curse), or which kills another person  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." 

The textbook for this class is The Dark Forces: A Guide to Self-Protection and can be purchased at flourish and blotts in diagon alley.  The Homework for this class is reading the textbook and writing reports on various dark creatures.  

1991 - 1992
Professor Slatero Quirrell
General Overview 
Quirrell's lessons were somewhat of a joke.  His classroom smelled of garlic and he had strange odors eminating from his turban.
1992 - 1993
Professor Gilroy Lockhart
Basic Dark Creatures
1993 - 1994
Professor Remus J. Lupin
Intermediate Dark Creatures
1994 - 1995
Professor Alistair (MadEye) Mooney
1995 - 1996
Professor Severus Quirrell

            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, but exist simply as a physical extension of an evil intention, they seek actively to harm.  Fantastic Beasts refers to many of them as Demons.
            The red caps are a great example. They bludgeon unususpecting travellers so that they can use their blood to stain their cloth caps red.  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.  Most of the time werewolvesthey aren't Dark Creatures, but when they are transformed, they attack people with the intention of passing the disease on to others or killing them.

                       Some Dark Creatures:
            red caps
            vampires (considered part-human)
            werewolves (although human part of the time)