Witch

Witches have red eyes and they cannot see very far but they have a strong sense of smell, like animals and can tell when humans are approaching.
Hansel and Gretel, The Brothers Grimm.

The second story of the series deliberately gets right away from vampires in order to reinforce the idea that Sunnydale is on a hellmouth and therefore will attract all sorts of supernatural activity. Joss and the crew start their exploration and defining of Buffy's world by going straight to magic. In order to begin all the many themes and circumstances planned for Buffy the fact of magic as real and alive had to be set up. Perhaps this was implicit in the fact that vampires exist but terms and rules had to be stated and the logical place to go after the intro episodes was to magic.
It seems by watching 'The Witch' that anyone in the Buffyverse can participate in spellcasting but that only certain people have an innate ability for using magic. We wouldn't call Giles a witch or warlock but he does have experience in using spells. Contrast his spellcasting with Catherine Madison's ability to use magic almost without thinking and it's pretty obvious that Giles is never going to be that good no matter how long he try's. Willow however might be and that marks the clear difference. There are those who cast spells from books and those who have the ability to wield power whether it comes from an external source or is their own.

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Catherine Madison