There are three kinds of symbol systems used in modern Japanese writing.
Kanji is the most common and oldest writing symbol. Kanji is originally from ancient China. Usually, one Kanji represents a meaning by its shape or stroke.
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Hiragana is usually used for a word which doesn't have an appropriate Kanji. Therefore, Japanese writing consists of both Kanji and Hiragana - because of this, you must have 100% accuracy, or your tattoo will look funny. >>See more about Hiragana
Katakana is derived from Kanji and is used mainly for translation of foreign languages, such as for the names of places, countries, a non-Japanese person's name, and so on. Katakana is also often used for representing the sound of something, like the sound of a dog barking for example. >>See more about Katakana