[Harusame]

Well, in Japanese, katakana is used for one main thing, and that's imported words. So like, their word for "printout" is written in katakana as "purinto-auto." There's a bunch of other words that are the same way. Those words I don't believe have kanji that they can be written with. Hiragana is used to write words that are of Japanese origin, but do not have a kanji. Also, I've seen in some video games, mostly for younger children, they seem to use more hiragana if they can, because younger children don't know as many kanji.

Hope that helps a bit.