Kanji is the term given to the use of Chinese characters, or hanzi, to represent the Japanese language. Kanji, along with hiragana and katakana, makes up the Japanese writing system. Additionally, Japanese may make use of some Latin characters, and may use Arabic numerals for representing numbers. Determining the exact number of kanji is somewhat problematic, but it is likely somewhere in the vicinity of 50,000 to 100,000 individual characters.
Kanji means, simply, characters of Han. Chinese characters came over to Japan first during the Han dynasty, sometime in the 1st century. Over the next few centuries, Chinese became used more and more in Japan. The Japanese language had no traditional writing system when kanji formed, and the early use of Chinese symbols would have been simply to write Chinese documents in Japan.
As the years passed, however, the Chinese symbols became tweaked to be able to handle the Japanese language. Small marks added to the letters, in a system called kanbun, told how the Chinese letters should be modified to have various particles and endings, to make them grammatical in Japanese. This was the direct predecessor to modern kanji.
A smaller set of Chinese symbols began to be used simply to relate sound, without any semantic value attached. This set was known at the time as man’yogana, and eventually evolved into hiragana. Similarly, katakana grew out of man’yogana, with monastic students transforming it to suit their own needs. In this way, both katakana and hiragana, the kana set, can be seen as descendents of kanji, rather than evolving concurrently with it.
Over the intervening centuries, the differences between kanji and hanzi became more and more pronounced. While originally the symbols were identical, they have since shifted. Alternate meanings have been given to most kanji, and the set itself was radically simplified in the period directly following World War II. During this period the Toyo Kanji Form List was introduced, creating much simpler forms for a wide range of words. Both forms are still in use, with the older ones known as kyujitai, and the newer ones known as shinjitai.
Kokkun is the term for kanji that retain the same general character as the Chinese, but have a completely different meaning. For example, the kanji symbol mori is the same as the Chinese sen. In Japanese the symbol refers to a forest, while in Chinese it means gloomy or majestic. Kokuji is the term for kanji which are unique to Japan, having no Chinese precedent. The term means literally “Chinese words created in Japan”. Examples of kokuji include the kanji for a tumbtack or freshwater catfish.
There are two main readings for any particular kanji. The first, the kun’yomi, is the Japanese reading of the character. The kun’yomi, or kun, is simply a reference to a Japanese word and pronunciation. The second, the on’yomi, is the Chinese reading. This roughly approximates the pronunciation of the word in Chinese, and there may be multiple ways to read the same kanji, even in the on’yomi reading, depending on the region and time at which the word was introduced to Japan.