Hiragana is used primarily to express words native to the Japanese language. Hiragana can be used to express vowels.
Katakana is used to represent foreign words, which could not be expressed using kanji. Katakana can be used to express sounds. Since the Japanese pronunciation of foreign words is frequently different than their native pronunciation, the katakana can represent something that sounds very different to those outside Japan.
Taken together, Japanese and Chinese symbols have become very popular in western society as artwork, tattoos and decor on household articles. While most people in the west have no idea of their meaning, their intricate design, especially kanji, is fascinating and appealing. Accurate translation of Japanese characters becomes necessary though, since many an unsuspecting westerner has displayed their new kanji tattoo, only to later find out that it means something entirely different.