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　　Kanji is an essential part of reading and writing in Japanese. Kanji is used for many nouns, adjectives, and verbs as well as all last names and most boys' names. In Japan, students learn various reading to several kanji in each year. They learn about 80 in first grade, 160 in second, 200 in third, 200 in fourth, 185 in fifth, and 181 in sixth totaling 1006 in elementary school. The last are taught in junior high. If they go to college after high school (which most do) they learn about a thousand more characters. To be considered literate, though, you only have to know the jouyou kanji or daily use; the ones taught in elementary and junior high schools. I might teach the jinmeyou, the ones taught in college, if I can find everything.

　　Kanji were taken from the Chinese hanzi around 300 AD. I've heard it was either by some Chinese (can't remember what they did) or Koreans looking to make a treaty with Japan against China. The Japanese emperor was so impressed with the writing, since they had never written anything down before, that he sent his son to learn Chinese. Soon all of the royal court spoke Chinese instead of Japanese.

　　On'yomi are Chinese readings, readings taken from the Chinese. They are usually used in compounds, explained later in this thread. Kun'yomi is Japanese readings, readings from the Japanese words. They are usually used when the kanji are standing alone in a word. the okurigana afterwards usually helps in knowing which of the kun'yomi to use if you know the word anyways. These rules do not always apply, however. e.x. 一 is pronounced as いち by itself, which is an on'yomi, but in the word 一人, 一 is pronounced ひと, which is a kun'yomi. It's best to learn the words after you learn the kanji or right along side. If you still don't know how to pronounce something when reading it out loud, say it one way and someone is sure to correct you.

　　All of the kanji are in the order the Japanese learn them separated by grade. Most have both on'yomi and kun'yomi, but some do not.