What is the difference between a Kantian Category and a Platonic Form?

The difference is that Plato's forms are specific, while Kant's categories are general. Let me explain. In Plato's theory, each and every object or quality that you can contemplate has its own ideal form. Corresponding to the experience of a horse there is the ideal form of a horse; and corresponding to the human conception of the good, there is an abstract form of the good...and so on and so forth. Everything in particular has its own perfect form. Not so for Kant, who actually was much closer to Aristotle in this. For Kant, there were certain categories that were GENERAL, in the sense that they applied to ALL objects across the board. To be an object at all, a thing had to possess, or be described in terms of, those categories. That's because those categories are so very general that ALL objects share them in common. And that's the basic philosophical difference.