Kanji are the adopted logographic Chinese characters that are used in the Japanese writing system. They are used alongside the Japanese syllabic scripts hiragana and katakana . The Japanese term kanji for the Chinese characters literally means "Han characters".
Most kanji characters can be pronounced at least two different ways according to whether the word is a native Japanese word (this is called the “kun” reading, or “kun-yomi”), or whether it came from Chinese (this called the “on” reading or “on-yomi”). Kanji that stand alone usually have kun-yomi, while kanji compound words usually have on-yomi. For example, on its own is "yama" (the Japanese word for “mountain”). When it is combined with another word or name, it is pronounced "san" e.g. “Fuji-san” (Mount Fuji).
However, there are many exceptions to these patterns. For example, some native Japanese words with two or more kanji (usually proper nouns) use their kanji's kun-yomi, like , which is pronounced "yama-guchi", even though the on-yomi for the kanji would be "san" and "kō" in that order. Sometimes, kanji only have on-yomi and have no kun-yomi. For example, the kanji , which means "poison", only has the on-yomi "doku". Sometimes, compound words mix on-yomi and kun-yomi together. For example, the word , which means "golden", is pronounced "kin-iro", in which the first kanji uses its on-yomi, and the second-kanji uses its kun-yomi. Sometimes, the kanji in certain compound words may not even use their kanji readings at all, but are used only to show the word's meaning. For example, the word is pronounced "tabako", which means "tobacco" in English. This word was borrowed from Portuguese, and while the pronunciation of the word does not match either kanji, the meaning of the kanji are "smoke" and "grass" in that order, hinting at the word's meaning. There are even some words whose kanji can be used to only show a word's pronunciation and not its meaning. These are called ateji. For example, the word , which means "America", is pronounced "Amerika", but none of the kanji meanings have any obvious connection to America. Usually, however, words of the last kind are usually written in only katakana these days, though sometimes words like "sushi" still written in ateji, like , because Chinese tourists will recognize the ateji more easily, since the word is written the same way in Chinese.
Kanji characters are classed on the basis of how many brush strokes they are made of. The simplest kanji have one stroke and the most complex may have up to 30 strokes.
There are many different Kanji, the exact number is not known but it is around 50,000. However, not all 50,000 are taught in schools. The government of Japan has set up a 1,945 basic kanji list (Jōyō Kanji) that those learning the language ought to know. They are the ones most commonly used in newspapers and magazines, along with street signs and store signs. Kanji takes many years to learn. By the end of his school years a pupil should know about 1,850 kanji. A university graduate would need to know about 3000. If someone does not know the kanji for a word, they can write it in kana and it will be understood, but it is usually not the proper way to write it.
Source: Wikipedia: Kanji