Hiragana is used for native Japanese words and grammatical elements, and katakana is mainly used for words of foreign origin :)

First answer by Alexander is correct. At first sight, hiragana appears more rounded and curvy, whereas katakana symbols are often sharp and pointy. From the perspective of someone whose maternel language is not Japanese, it can be said that katakana is a curse, discriminatory and detrimental to the integrity of the japanese language itself (a point on which I will elaborate in a separate thread). For example, when foreigners residing in Japan go to the town or city municipal office to register their personal seal (判子　hanko, or 印鑑、inkan)　which is obligatory for opening bank accounts, entering into work , business or marriage contracts, buying cars or houses etc, we are forbidden from registering seals with kanji (chinese characters) or hiragana, simply because we are not Japanese. We are told that we can use only the limited, and less attractive katakana symbols, which also often fail in producing the correct pronunciation of our names. Last year, I tried to persuade some robotic, inflexible city office worker that there was no basis in law or logic for this outdated rule. He just said , 'You are foreigner, must use katakana'... unimpressed , I had no choice but to pay again to have another hanko made. That katakana was ever developed and introduced in the first place, hundreds of years ago, gives one some understanding of how impure and inferior all things foreign were deemed to be by Japanese people in those times, unworthy of being described or written about, in hiragana, lest they contaminate the language itself. (how things have changed!)

Annyeong-haseyo! (Are you Korean?) Unlike Hangeul, japanese uses 3 scripts namely HIRAGANA, KATAKANA and KANJI. KANJI is essentially same as HANJA... for example "namja/남자/男子" is "dango" in japanese Katakana is used to write foreign loan words...Hiragana for everything else (except those that are written in Kanji)