The American Needlepoint Guild has an inclusive definition of the craft: "Any counted or free stitchery worked by hand with a threaded needle on a readily countable ground."
A needlepoint pillow kit usually comes with all the materials for the front of the pillow but does not include the fabric for the back or the stuffing or pillow form you will need.
The other major difference between cross stitch and needlepoint is that a needlepoint design fills up the entire fabric, while cross stitch can be large or small but almost always has blank canvas around the stitched image.
Stitching.com has a great free online needlepoint guide, which shows how to work stitches on different kinds of canvas, how to work a project, and some of the basic stitches.
While there aren't a lot of things you need to buy to start working with needlepoint, you'll be glad you took the time to invest in some good needlepoint supplies before you begin your first project.