Hard to describe but they are very different. I did just post links to photo tours of Portland's Chinese and Japanese gardens on the terrestrial gardening section. If you look through them you will see they are very different.
I don't know about all of them but here the Chinese garden is walled and smack dab in the middle of the city and has a lot of rocks, like the ones in the photo above, very different looking. Lots more hardscape too.
The Japanese Garden's rocks are mostly smooth and natural like a stream polished them. There are few hardscapes.