Is it their practices that sets them apart?
Yes. Everything else, no. Darker magics is just a moral judgment and practically anybody can do rituals or make deals with Daedra. The difference seems to be in their attitude to the whole matter, not with what they do with magic.
Turala turned to Cassyr, who shook his head. "Witches. Am I correct in assuming that this is Old Barbyn's Farm, the home of the Skeffington Coven?"
"You are, pet," the old woman giggled girlishly, pleased to be so infamous. "I am Mynista Skeffington."
"What did you do to those orcs?" asked Turala. "Back there in the thicket?"
"Spirit fist right side the head," Mynista said, continuing the climb up the hill. Ahead of them was the farmhouse grounds, a well, a chicken coop, a pond, women of all ages doing chores, the laughter of children at play. The old woman turned and saw that Turala did not understand. "Don't you have witches where you come from, child?"
"None that I know of," she said.
"There are all sorts of wielders of magic in Tamriel," she explained. "The Psijics study magic like its their painful duty. The battlemages in the army on the other end of the scale hurl spells like arrows. We witches commune and conjure and celebrate. To fell those orcs, I merely whispered to the spirits of the air, Amaro, Pina, Tallatha, the fingers of Kynareth, and the breath of the world, with whom I have an intimate acquaintance, to smack those bastards dead. You see, conjuration is not about might, or solving riddles, or agonizing over musty old scrolls. It's about fostering relations. Being friendly, you might say." - http://www.imperial-library.info/mwbooks/2920.shtml
» Sun Feb 06, 2011 1:32 pm 
