Kana itself consists of a pair of syllabaries (writing systems in which each symbol represents a syllable): hiragana, used primarily for native or naturalized Japanese words and grammatical elements, and katakana, used primarily for foreign words and names, loanwords, onomatopoeia, scientific names and sometimes for emphasis.
Almost all written Japanese sentences contain a mixture of kanji and kana. Because of this mixture of scripts, in addition to a large inventory of kanji characters, the Japanese writing system is often considered to be one of the most complicated in use anywhere in the world.
The co-existence of several writing systems in Japanese has a historical explanation.
Literacy was introduced to Japan in the form of the Chinese writing system before the 5th century.
At first, the Japanese wrote in Classical Chinese, with Japanese names represented by characters used for their meanings and not their sounds. Later, during the 7th century AD, the Chinese-sounding phoneme principle was used to write pure Japanese poetry and prose, but some Japanese words were still written with characters for their meaning and not the original Chinese sound. This is when the history of Japanese as a written language begins in its own right.
Over time, a writing system evolved. Chinese characters (kanji) were used to write either words borrowed from Chinese, or Japanese words with the same or similar meanings. Chinese characters were also used to write grammatical elements, were simplified, and eventually became two syllabic scripts: hiragana and katakana.
Hiragana and katakana were first simplified from kanji and hiragana, emerging somewhere around the 9th century, was mainly used by women. Hiragana was seen as an informal language, whereas katakana and kanji were considered more formal and were typically used by men and in official settings. However, because of hiragana's accessibility, more and more people began using it. Eventually, by the 10th century, hiragana was used by everyone.
Several thousand kanji characters are in regular use, which mostly originate from traditional Chinese characters. Others made in Japan are referred to as “Japanese Kanji”. Each has an intrinsic meaning (or range of meanings), and most have more than one pronunciation, the choice of which depends on the context. Japanese primary and secondary school students are required to learn 2,136 kanji as of 2010. The total number of kanji is well over 50,000, though few if any native speakers know anywhere near this number.
In modern Japanese, the hiragana and katakana syllabaries each contain 46 basic characters or 71 including diacritics. With one or two minor exceptions, each different sound in the Japanese language corresponds to one character in each syllabary. Unlike kanji, these characters intrinsically represent sounds only; they convey meaning only as part of words.
Hiragana symbols are used for words without kanji representation, for words no longer written in kanji, and also following kanji to show conjugational endings. Because of the way verbs in Japanese are conjugated, kanji alone cannot fully convey Japanese tense and mood. For this reason, hiragana characters are appended to kanji to show verb and adjective conjugations. Hiragana can also be written in a superscript called furigana above or beside a kanji to show the proper reading. This is done to facilitate learning, as well as to clarify particularly old or obscure (or sometimes invented) readings.
Katakana symbols are primarily used to write foreign words, plant and animal names, and for emphasis. For example, "Australia" has been adapted as Ōsutoraria (オーストラリア), and "supermarket" has been adapted and shortened into sūpā (スーパー).
The Latin script (or romaji in Japanese) is also used to a certain extent, such as for imported acronyms and to transcribe Japanese names, and in other instances where non-Japanese speakers need to know how to pronounce a word (such as "ramen" at a restaurant). Arabic numerals are much more common than the kanji when used in counting, but kanji numerals are still used in compounds.