You could try thinking of the Kana as the alphabet of Japanese - hiragana representing lower case and katakana upper case, in a funky kind of way. Or perhaps it's better to think of katakana as italics. Anyway, they describe the individual sounds in Japanese. The kanji on the other hand are sort of like words, or parts of words. And so you can write out the sounds of the kanji using kana, but not (really) the other way round. Well, start learning and you'll soon work it out.
Kana was developed by the Japanese and have no relation to Kanji what so ever.
I agree with most of what you said, djadams, but I can't go along with this statement; they have quite a close relationship in my opinion. For example, just look at how the following hiragana were developed from kanji:
安 into あ
幾 into き
奈 into な
美 into み
波 into は