Dark magic is used to inflict harm on others. A very common example is creatures who eminate magical fear along with Muriel, who is a user of dark magic. Dark magic users aren't nessesarily all bad, as seen with Seth Sorenson, he is a Shadow Charmer so therefore is a user of Dark Magic, but uses it for completely good purposes such as to help the Knights of the Dawn.
Dark Magic is what separates Dark Creatures from Light Creatures. They are users of Dark Magic and are born dark, unless turned dark by the Shadow plague.