On-yomi and Kun-yomi (Romaji version)
This is the same as the previous post, only with things changed to romaji for beginners that haven't fully mastered kana yet.
When referring to kanji, On-yomi (おんよみ) and Kun-yomi (くんよみ) are two kinds of readings in Japanese, the former being the intended Chinese pronunciations (in Japanese, of course) of the character and the latter being the adapted Japanese pronunciations of the character. Some characters may have very few readings whereas some may have a lot of them.
Generally, On-yomi are written in CAPITAL LETTERS (katakana in kana), and Kun-yomi are written in lowercase letters (hiragana in kana), the On-yomi usually having some sort of break to separate the kanji from the extra kana involved (called okurigana (おくりがな)).
Take the kanji 好 (English meaning: fond, liking), for instance. This kanji is read as KOU, kono(mu) and su(ku).
Another example would be 行 (English meaning: going), which is read as KOU, GYOU, AN, i(ku), yu(ku), and okona(u)/oko(nau).
The On-yomi of a kanji is usually seen in kanji compounds (called jukugo (じゅくご) which I will get to in a later post) while the Kun-yomi are either seen in verbs or in Japanese readings of a word.
There are literally thousands of kanji in the Japanese language, but fortunately, one needs to know 1,945 kanji to get by in Japan. Still, it is impossible to try and cram ALL of the readings for ALL of the 1,945 common kanji (called Jouyou Kanji (じょうようかんじ)) in one sitting, so it's best if you only learn a few essential readings of a kanji and learn the rest of the readings through context. After all, it'll probably be a while before you run into AN as a reading for 行, right?