I believe the "all you care to eat" is the more polite reference. I guess "all you can eat" suggests totally pigging out! They both mean the same thing however - one price for dinner and refills on the food if you'd like.
They don't necessarily refer to buffets, although most Disney "all you can (or care to) eat" places are buffets. There are a few all you can eat meals served family style - Whispering Canyon has one, the character breakfast at Ohanas. And Cindy's is plated, but I think its still "All you can eat"