Spell Casting is the power to work magic through the recitation of incantations and words of power. It is the most common magical ability employed by witches.
Ranging from summoning and sending-back spirits to enacting curses, the capabilities of spell casting are virtually unlimited, though it likely depends on the magical skill and experience of the witch. While spell casting may be used on its own, the magical potential will be greater when used in conjunction with external aids such as herbs or crystals.
Since most spells need to be spoken to take effect, spell casting may be ineffective if the witch is verbally unable to do so. There are also factors that should be taken into account when spell casting is used (such as items needed to make the spell active), thereby time and concentration should be taken into account. This could leave the witch vulnerable to outside interferences, as seen when Thea needed Eric to distract Suzanne Blanchet while she was casting the sending-back spell.  It can also depend upon the power and skill of a witch, and the correct incantation; for example, the untrained witch Gillian Lennox was able to successfully curse Tanya Jun and Kim whilst acting under Angel's instructions, but when she first attempted to undo the spell, she didn't know the correct words and so it failed to work.