The short answer is Yixing and zisha are commonly used interchangeably to describe the same type of teapot. As others have mentioned, Yixing is a city famous for producing these pots, and zisha is a term used for the material. However, zisha (zi = purple, sha = sand) is used both
as an umbrella term for all clay from the Yixing area, OR a specific type of fine, high quality purple clay within the family of Yixing clays.
It can be a little confusing at first.