Haven't used Rosetta Stone but I have been studying Japanese for a few years now. They use 3 alphabets, kanji, hiragana and katakana. Together they are called Kana. Kanji and Hiragana are used to write native Japanese words and Katakana is used to write foreign words. You need to learn all three to be fluent in Japanese.
Kanji is made up of Chinese characters that have been adopted into the Japanese language. Thankfully there are a lot fewer kanji characters in Japanese than Chinese, but you still need to memorize about 2000 kanji to be considered fluent in the language.
Hiragana is a syllabary, which means that each character in Hiragana represents one mora (similar-ish to syllables), except for the n character. All japanese words can be written using Hiragana and while you are learning kanji you'll probably use it a lot for words with complex kanji characters.
Katakana is probably the easiest alphabet in Japanese, it's used for all non-japanese words. It's actually not used that much by people learning japanese (since they are just working with japanese words) unless they have to write down their name, or something non-japanese.
EDIT: Japanese can be written in romaji (the latin alphabet). For example;
今日は
こんにちは
Konnichi wa
^ "Good day" written in Kanji, Hiragana and romaji