japanese language has 4 alphabets. Hiragana is the first, used for words that are from Japanese origin, such as おはようございます。(Ohayou gozai masu, aka formal 'good morning'.) Katakana is for western terms that are not originally japanese but have been translated/migrated into the japanese language, such as アトム ('Atomu', japanese localized form of Atom) or アメリカ ('Amerika', japanese localized form of America.) I don't know much about kanji, but I know it is often mixed with katakana and hiragana. I think kanji are more complex characters that stand for a whole word, while hiragana and katakana alphabet stand for sounds or segments that make up words when put together. The fourth alphabet is romaji, which is spelling japanese words with english characters, basically. such as above, where Ohayou gozai masu is the romaji form of おはようございます。

If you want to know so you can only learn one and use it, that might not work. Besides, kanji is mixed within hiragana and katakana sometimes. Japanese, for some reason, like to use katakana and hiragana whenever they want to...