Source: NY Times.
It'll be based on adaptations of her "Fantastic Beasts and Where to Find Them" book. However, it will not be a prequel or sequel of the Potterverse.
Essentially, "Fantastic Beasts and Where to Find Them", as if you don't already know, is sort of like a guidebook to the world Harry lives in. Think of it as a companion book and money printer for the Rowling enterprise.