This is a job used by the wizards and witches to create new spells for the wizarding population. Generally they create the rules of how to preform the spell and design the langauge usage and pronounciation. This is a highly dangerous study because spells can go wrong or can be created for dark purposes. A legal job generally makes spells that are used or legalized by the Department of Magical Affairs.
Its also a class offered in Spellmaster University.