Joss's Whitch theroy
There is a desisive diffrence between how whitches are portrayed in seasons 1and 2 and then the latter season. In the first two seasons Joss went for the traditianl Whitches are bad thay try to hurt people and they have to be born whitchs. This is showed by Amys mother in the episode "The Whitch"
However things change when willow starts to study the black arts and is also called a whitch. this is towards the end of season 2 and season 3 onwards. Jenny calender is a big influence in this.
As well as performing spells many "Whitchs" are also part of the religion of Wicca. this religion is based of women in contect with nature. although many of them think they are Whitches. Willow is disapointed when she attends a Wicca group and it appears there are no other actual whitchs there. although it dose turn out that there is one. Tara. Willows future love intrest
Inthe Buffyverse Whitchs have to chant spells to utilise Magic and it is not a power they own.