A lot. While there's many similarities between Chinese & Japanese culture and food (mainly because Japan was heavily influenced by China, Korea, and other countries), they differ a lot.
Pork, Duck, Chicken, and fish are the most common poultry in Chinese cuisine (and I don't mean Americanized Chinese food like "fried rice" or something), where as in Japanese, it's mainly seafood. Chinese don't really like to waste their food, so to be a bit blunt, they eat everything. The skin is eaten along with the meat, the bones are used to make broth, etc. Also, Chinese food tend to be a bit more greasier, where in contrast, Japanese food is much cleaner to the palette(sp?), and not as heavy. But it depends on the food you're eating. There's a lot of non-greasy foods in both Chinese & Japanese cuisine.
To me, it's hard to tell, between Chinese and Japanese cuisine, but my parents say that Japanese like to BBQ everything. As in grilling. i.e. - Yakitori, Okonomiyaki, and Teriyaka.
But Chinese does its fair bit of BBQ-ing as well. For example, Char siu. It's barbequed pork, and it's actually eaten in Japanese cuisine as well, such as Ramen. Though as far as my knowledge goes, Chinese cuisine usually prepared by stir-fried, roasted, steamed, or boiled. (Japanese have similar different ways of preparing their foods, that I honestly am not familiar with.)
Also, Japanese cuisine include raw foods such as shishimi. Chinese, on the other hand, do not usually eat foods that are raw. At least, that's how it is for me and from what I've read about so far.
This answer may not have been very helpful. Sorry. I'm Chinese and I grew up around Chinese cuisine so I do not know much about Japanese cuisine. :)