The way I've heard it, Hiragana is the "everyday, general-use alphabet". Katakana uses the same spoken syllables as Hiragana, but the characters are written differently because it (Katakana) is used exclusively for foreign (non-Japanese) words. Kanji is used for specific words, particularly proper nouns (specifically names of people and places) and is based on Chinese calligraphy, which was what the Japanese were using before they developed/came up with Hiragana.