The difference is in use. Hiragana is the standard character set while Katakana is used in more specific ways. Katakana is used mostly for using text to describe sounds (not spoken language) and foreign words (including those adapted into the Japanese vocabulary). They may also be occasionally used to emphasize certain words by the writer or can be used for spelling the names of plants and animals.

Mytch is almost right. Hiragana is the standard syllabary. Katakana is used where in English we would use italics. Its other use is for designating "loan" words, e.g words taken from another language. In truth, katakana isn't really necessary and my previous comment about redundancy still stands.

Becasue the Japanese are masters in the art of redundancy! That's the only answer that ever made sense to me.