Presentation:
CJK is the abbreviation of Chinese-Japanese-Korean, three geographically close languages. They all use hanzi (C) or kanji (J) or hanja (K), the chinese characters, also called sinograms or ideograms. These languages always attracted me...

Note : we call romanization the translation of foreign language in our alphabet, (the roman alphabet: a, b, c, d...).

My research:
Chinese, also called mandarin in the West, zhongwen when dealing with written Chinese, or putonghua, hanyu, zhongwen, guoyu or zhonguohua for spoken Chinese, is the most spoken language in the world, (900 millions people would use it) before English (450 millions). It is composed of several thousands of characters (at least 56,000 have been counted), 3,000 being regularly used. Each character has its own pronunciation (a syllable and a tone, in Chinese there are approximately 400 syllables and 4 tones), it is maybe the main difficulty of this language, some characters having several pronunciations. A character often represents an idea, a concept, and associated with other characters forms words. These associations are sometimes semantic, sometimes phonetic. A character is composed of radicals (there are approximately 300 radicals, 100 are frequently used). Since 1954, China has begun to rationalize these characters by simplifying their writing. Nevertheless, the old characters (called "traditionnal" and opposed to the "simplified" ones, the simplified characters being now the official ones in mainland China) are still used in Hong-Kong and Taiwan.
[romanization] The most used romanization system of Chinese is pinyin. It has been adopted by China in 1958, internationally in 1979. But, in French-speaking countries, the system of Ecole Française d'Extrême Orient (EFEO), used since 1902, is sometimes employed because closest to the origonal pronunciation. In English-speaking countries, the Wade-Giles system was used before pinyin. In Taiwan, Tongyong Pinyin (or Ziran Pinyin) and bopomofo (Zhuyin Fuhao) are used.
Table of Chinese radicals
Table of the various romanization systems of Mandarin

Note: Cantonese, or guangdonghua, is the most famous dialect of China. Spoken by approximately 50 millions of people in China, it is also spoken abroad, among "Chinatown" expatriates. Cantonese differs from Mandarin mainly in its pronuciation (the writing part is very similar, despite some sentence structure or character choice differences), it has 9 tones even if romanization systems only kept 6 of them. Cantonese has no official romanization system, several systems can be found: Sydney Lau, Yale, Jyutping, (Meyer-Wempe) etc.
Table of the various romanization systems of Cantonese

Japanese, also called nihongo, is the 10th language in the world, with 122 millions locutors, just before French (120 millions). This language uses three alpahabets: hiragana (simple smooth characters - 46 syllables), katakana (simple straight characters - 46 syllables) and kanji which are actually Chinese characters, globally less numerous than in Chinese: there are 1945 official kanji called jôyô kanji. The difficulty of learning Japanese comes from the several pronunciations of each kanji: at least, each kanji has a "Chinese" pronunciation ("on-yomi") more or less similar to the real Chinese pronunciation, and a Japanese pronunciation ("kun-yomi") coming from Ancien Japan before Chinese characters were used. Sometimes a kanji has several on-yomi & kun-yomi readings. On the other hand, contrary to Chinese, Japanese has no tone.
[romanization] The most famous Japanese romanization system abroad is the modified Hepburn system. Japanese prefers to use the Kunrei system which is slightly different from Hepburn on specific sounds: shi/si, ji/zi, chi/ti, tsu/tu, fu/hu. Also, the Wâpuro system, close from the other ones, is used in computer science.
Table of hiragana & katakana alphabets, and romanization systems

Korean is the 15th language in the world with 72 millions people speaking it, before Italian (63 millions). Korean used Chinese characters (hanja) but they are now rather restricted to litterary Korean. In 1446, an alphabet (Hunmin jeong-eum) had been created very scientifically, and in 1912, slightly reviewed and re-called Hangul or Hangeul. It is a 24 radicals (jamo) alphabet, with 10 vowels & 14 consonants, which allow to build characters. As Japanese and contrary to Chinese (vietnamese...), Korean has no tone.
[romanization] Until July 2000, the romanization system of Korean was the McCune-Reischauer (MR) system. But the "National Academy for the Korean Language" (NAKL) decided to make some recent changes (controversed but official) in order to avoid ' and accents on two vowels.
Table of Hangul alphabet and romanization systems