Kanji is literally "Chinese character". They are ideograph; each letter represents a meaning. A select number of characters were used to represent the syllables in Japanese (known as gojūon). They evolved into what is known today as hiragana and katakana (both of them we group as kana).
- Hiragana: forms the basis of Japanese grammar particles, which may include prepositions and conjunctions, etc. Words of local origin are usually written in hiragana.
- Katakana: Is used mainly for non-Han (Chinese) loanwords, like words introduced from English, Dutch, German, Korean, etc. Also sometimes used to denote emphasis. Is also used for showing the reading of kanji, esp. for proper nouns (like to denote how your kanji name is pronounced).
- Kanji: is mostly used for words of Han origin, these make up most of the Japanese lexicon. May also be used for ateji: representation of words using ideographs.
The qualities make Japanese writing system somewhat unique from many other languages.
If used outside of the above context, hiragana usually has a softer nuance than katakana, so sometimes parents name their daughters using only hiragana.
Using katakana for something usually written in kanji/hiragana may give it some impact.
Road gangsters like to use kanji with violent meanings to replace kana of similar sound. For example, the word よろしく (yoroshiku) is often found in graffitis as 四露死苦 (ideographs for four, mist, death, and suffering. Note that four has a meaning of death in east Asian superstition)