In reading about the 4 Satipatthana or about the 5 Aggregates, it seems the first category of each (form) is sometimes called rūpa and sometimes called Kāyā. Are these just two words for the same concept or are there subtle differences? Thank you.
Rupa is "shape", "form", "object", "appearance", "image", usually in the sense of something perceived visually.
Kaya is "body", both specifically of living being, but also generally a body of a tree (i.e. trunk), a body of a guitar (i.e. sound box), and even a body of a teaching (dharma-kaya).
Although in Theravada there is a tradition of using the word rupa to refer to "material" aspect of a living being, I would say "rupa" is more frequently used in the phenomenological sense ("what is perceived"), while "kaya" has more ontological flavor ("what is").