The Japanese alphabets are based upon the Chinese alphabet. The Kanji characters are taken straight from Chinese, and the Katakana and Hiragana alphabets are simplified versions of the Kanji characters.
Kanji have two types of pronunciation, one which is based on the original Chinese pronunciation of the character. The meanings of the Kanji in Japanese and Chinese are usually similar, but because the Japanese alphabet was created so long ago, the meanings have changed a lot and often the nuances are very different.
Other aspects of the languages are mostly dissimilar: the grammar, pronunciation, etc.