Firstly, the Japanese Language and the Chinese Language is not of the same linguistic family, thus there will be a large number of differences.
Japanese consists of syllables called mora and consists of a consonant cluster plus a vowel. Several mora make a "word" in Japanese. In contrast, Chinese is broken down by character, which each character is given at least one reading of exactly one syllable long. Chinese also distinguishes between tone in all its dialects; Japanese does not and uses a tone-based stress to clarify what is being said.
It is in this manner the Japanese developed kanji, the Chinese characters used in Japanese.
Japanese uses three different scripts, not counting Roman Letters and Arabic Numerals, in everyday writing: kanji, hiragana and katakana. Kanji, as noted, is the Chinese characters seen in Japanese text. Hiragana and Katakana are symbols derived from grass script calligraphy of Chinese characters and evolved to be their syllabary. Chinese only uses Chinese characters.
Japanese also imported a large amount of vocabulary when importing Kanji from the Chinese. This gave two results: many distinctively Chinese concepts retained their Chinese reading, while many of the Japanese concepts gained a kanji which meant what the kanji represented in Chinese. Aside from these similarities, there is almost no point in common between the vocabulary of the two languages.
Japanese and Chinese employ two completely different grammar schemes - first, even the order of the sentence would not be the same in the two languages: Chinese is mainly Subject - Verb - Object, like English (Although it is technically possible to construct a sentence meaning exactly the same thing using a different order) and Japanese is always Subject - Object - Verb, like German most of the time. Japanese also possess a past tense (but no future tense) while Chinese does not distinguish the tense at all.