Japanese phonetic lettering system
Unlike Kanji, which is made up of ideograms or pictograms where the characters are abstract representations of actual objects or concepts (ie. drawings), Katakana is a purely phonetic character set where each "letter" is intended to represent a sound. It is different from the other Japanese phonetic character set, Hiragana, in that its intended use is for loan (foreign) words. While it is popular sometimes in Japanese youth culture to use katakana on Japanese words to imply emphasis, this is not a grammatically correct use of katakana and is not recommended for tattoos unless you are familiar with the Japanese language.
While katakana is much easier to work with for a beginner of the Japanese language than kanji, those interested in getting a katakana tattoo are still advised to consult a Japanese friend beforehand. Most loan words (and names!) have standardized katakana spellings and may not correspond to the phonetic pronounciation. Here, a basic Japanese sentence where Katakana letters are in green, Hiragana letters are in black and blue (blue are sentence particles) and Kanji in red: