Well ... they sound different ... but if you are talking about the writing.
Chinese is all in "complicated" characters ... Japanese has some of these but not as many in everyday use as Chinese. Chinese has over 50,000 complex characters but most Chinese really only need to know about 8000-10000. Japanese has a couple of "alphabet" systems call kana, and some complicated characters called kanji as well, and knowing 2000 kanji is probably enough to get by, but some people know probably up to around 10,000. Japanese has large dictionaries of kanji that are about as large as the ones that china uses for their characters.
The languages are in related families, but are different. Like English and German, or French and Italian. Some the complex characters have the same meaning in both Chinese and Japanese, but many look the same but mean very different things.