hiragana and katakana

[omryomry]

do i have to learn katakana to know japanese well? can't i use hiragana and kanji only?
and what's the difference between hiragana and katakana except for katakana is for foreign words?

[Shirasagi]

To know it well? Um, yeah, it's a basic skill. In addition to foreign words, it's often used for flora and fauna, for effect, for slang, and in lieu of hiragana in some pre-war writing. It's pretty ubiquitous.

[Teh_Freak]

You don't need to know any of the writting systems to know Japanese. Unless you plan on reading and writting that is.