kanji 漢字 is the Japanese term for the Chinese characters. hanzi in Chinese and hanja in Korean.
Currently, three systems of kanji are used in Asia:
- Traditional (ex. 吳清源): Used in Taiwan, Hong Kong, Macau and Korea.
- South Korea largely abandoned use of kanji in the mid 1980s.
- Japanese New Kanji (ex. 呉清源): Somewhat simplified version, used from the late 1940s in Japan. About 70% of them match the Traditional forms.
- Simplified (ex. 吴清源): Greatly simplified version. First used in Mainland China in the 1950s, later adopted by Malaysia and Singapore. About 60% of them match the Traditional forms.
Examples of different forms (Traditional/Japanese/Simplified)
In Japanese, the same character can have different pronunciations and/or meanings depending on the context. (Example: 中 (naka) in 中手 (nakade) and 中 (chuu) in 中盤 (chuuban).) In other countries, a kanji has only one pronunciation, though there are regional differences. For example, 中 is pronounced 'zhong' in Chinese and 'jung' in Korean.
Some kanji characters can have more than one form, often different arrangements of the same elements. Example: 峰/峯 are two forms of the same character.
A tricky case is 棋 and 碁. In modern Japanese, these are different characters and used in the names of two different tournaments, 棋聖 (Kisei) and 碁聖 (Gosei). However, in Chinese, these two are considered as different forms of a character pronounced qi. Therefore the Japanese Gosei tournament is often referred to as shao-qisheng (小棋聖, small Kisei) to distinguish it from the more prestigious Kisei tournament.
To learn more, see Halpern's outline of the Japanese writing system.
See also Benjamin Barrett's multilingual go dictionary for pretty much every go term.