1. hiragana (ひらがな): A syllabary with 46 characters representing syllables. It is used to write grammatical particles, verb endings, and words that don't have kanji.
2. katakana (カタカナ): Another syllabary with 46 characters representing syllables. It is used for foreign words, onomatopoeia, emphasis, and sometimes for proper nouns.
3. kanji (漢字): Logographic characters borrowed from Chinese, each representing a word or concept. Kanji are essential for understanding Japanese, as they convey meaning and often function as the core of a word.
It's common to think of romaji as the fourth writing system. However, romaji is actually a romanization of Japanese using the Latin alphabet. It's primarily used for pronunciation guides and for entering Japanese text on computers that don't have Japanese input capabilities.
Therefore, while romaji is important for understanding Japanese, it's not a true writing system in the same way as hiragana, katakana, and kanji.