Napa cabbage is what koreans use to make Kim Chi. Different from regular cabbage in america, which is 양배추.
Well there are two kinds of cabbage that you usually can get in the US- the regular spherical one (which we just call cabbage) and then theres napa cabbage which is longer and less round.
Maybe this is only among people who cook, but i at least distinguish between them
Interesting, in Australia we call 'Napa cabbage' as 'Wombok'. First time ever hearing the former in my life and I cook regularly :o