Kanji are symbolic Chinese characters used to write the Japanese language. Japanese supplements kanji with the phonetic scripts of hiragana and katakana, but kanji is the basis of written Japanese.
History of Kanji
Japanese was purely a spoken language before the introduction of kanji from China, via Korea, in the 5th and 6th centuries A.D.. The main use of kanji in Japan at that time was for official diplomatic documents and Buddhist texts. The art of writing those characters, i.e., calligraphy or shodo in Japanese, became essential for learning, and is still a required subject in Japanese elementary schools.
But Japanese is a completely different language from Chinese, so two supplementary phonetic "alphabets," hiragana and katakana, were developed from kanji to ease along what was essentially a difficult marriage between the Chinese writing system and the Japanese language.
Kanji Radicals 部首
The building blocks of each individual kanji are ideographic elements known as radicals, or in Japanese bushu (部首), of which there are 214. Many kanji comprise a single radical, however most are a combination, with each radical taking one of fourteen possible positions, such as top, bottom, left, right, corner, enclosing, etc.
The general meaning of a kanji can generally be understood by recognizing just its main radical. For example, the radical "gold" 金 - typically at the left of a character - signifies that the meaning of character is associated with something metallic. Or the radical for "grass" - typically at the top, such as in "flower" 花 - denotes lesser vegetation, while the radical for "tree" 木- typically at the left - a tree, or at least something usually made of wood.
Kanji are listed in the dictionary by stroke number, i.e., how many lines or dots it takes to write it. A single stroke is usually just a line or a dot. A kanji dictionary begins with the kanji for "one": a single horizonal stroke: 一. and ends with the incredibly complex 23-stroke kanga[miru] 鑑[みる] (meaning "to heed" or "be mindful of").
In China, each kanji has only one pronunciation, but in Japan things got more complicated as a result of having to superimpose this imported system on the very different Japanese language. Japanese kanji retain the original Chinese pronunciation (or "reading"), which is called on yomi and identifies the kanji itself. However, in a sentence it is often read a completely different way, using, of course, a Japanese word that existed from before kanji entered the picture. This native Japanese reading is called the kun-yomi.
This sort of equates to what we call the letters of the English alphabet as opposed to how we actually pronounce them. For example, we call "C" "see," but often pronounce it like a "K," and we call "W" "double-you" but pronounce it completely differently, by pursing our lips and forcing a little air out. In other words, the Chinese-inspired name of the kanji and how it is pronounced in a Japanese sentence are often completely different.
To make things even more confusing, there is often more than one on-yomi because kanji were introduced from different parts of China at different periods, and reflect those differences. And there is often more than one kun-yomi, because, as in most languages, one word gets to acquire several different meanings (such as the English word "set", which has 464 definitions!)
One example is the word for "see" in Japanese: miru, written using the kanji: 見 mi and the phonetic hiragana る ru. So, miru is the kun-yomi, i.e., the native Japanese reading. But let's try combining that character for "see" 見 with the character for "things": 物. Now we have the word for "sightseeing," but its pronounced kenbutsu. Because it's a kanji + kanji combination, the native Japanese kun-yomi mi gets replaced by the Chinese on-yomi of ken.
Joyo Kanji 常用漢字
Japanese school pupils have to master the basic set of 2,136 Chinese characters called the joyo kanji, or "common-use characters." By the end of their six years at elementary school, students should know 1,006 of them (called the kyoiku kanji, "educational characters"), and by the end of three years of junior high school, a further 1,130. Knowledge of a further 500 to 1,000 kanji is needed to fully understand Japanese newspapers and magazines. These are acquired at senior high school.
Many Japanese kanji were somewhat simplified after the Second World War, and are called shinjitai (新字体 "new character forms"), but not to the extent that they have been been simplified in modern-day China.
Kanji characters are input using the hiragana or romaji keys of a keyboard, Typing a phrase or word will bring up a number of kanji suggestions to choose from.
Some useful books for getting started with kanji are :
The Original Modern Reader's Japanese-English Character Dictionary
The Kanji Dictionary