A spell is a series of words that when said in a certain order causes change in the fabric of reality to varying extents. It is a ritual intended to produce a magical effect. In order to be effective, it must be performed by a witch; a spell recited by a mortal will not work.
The act of performing a spell ritual is known as "casting the spell". Witches traditionally record their spells in their grimoires, both for their own reference, and to preserve them for posterity.
Spells can be non-verbal and can also be channeled and used by powerful telepaths. Protection spells are impossible to create, even if it keeps the person walking and talking technically the individual would still be dead, but unable to move on because the spell is holding them back.
Dark witches cast more powerful spells, but the results of the spells can be disasterous as dark witches can not focus the spells as well as white witches.
Some spells require more than one witch to make them work in order to increase the amount of magic and therefore potential power available for the spell. The spell against Adrian Kristoff and his dark coven took the combined powers of the entire coven, including Allison and Ayden's dark magic.