Well I am not quiet familary with the MAIN difference, when it comes down to both languages, you have to be specific you can't be general. Because in some parts both languages are similar yet differ at same time. Maybe this might help you:
Historically, the Kanji Writing System was imported from China, but as time the time went by, kanji writing has evolved into a part of the Japanese Writing System by using the borrowed ideograms to indicate pronunciation.
Although we use Kanji characters as much as Chinese people use, Japanese people use less Kanji characters comparing to Chinese people.
Even if you look at some of the Japanese culture, there are some adoption of cultural and religious materials from China too.
For instance, Chinese herbal medicine is quite popular in Japan.
--credit to:
http://www.learn-japanese-kanji-hiragana-katakana.com/JapaneseKanji_ChineseKanji.htm
That is one of them, theres way more, but you have to do some reasearch lol.
