What is furigana?
Small kana above kanji(漢字) telling the reader how those kanji are pronounced.
漢 is pronounce "かん" and 字 is pronounce "じ"
In html furigana can be reproduced by use of the ruby tag, however this tag remains unsupported by standard installations of many browsers including Firefox and Mozilla. The ruby tag was made part of the XHTML 1.1 specification on May 31, 2001.
You can download a furigana ruby extension for firefox from the official site
How is furigana used?
As well as the "official" use described above furigana is also used in a number of other ways.
- To give the meaning of a word in contrast to what is actually said.
For example あの人 could have the furigana of おんな above the kanji 人 to make it clear that it is a woman being talked about.
- To give what is said in contrast to the exact meaning being referred to.
For example you might have 義妹 with the furigana いもうと when いもうと is what is being said but the person in question is actually a step or adopted sister.
- With カタカナ語.
Particularly in manga it is common to have some term said in "English" (written in katakana) but with the Japanese word given (often in kanji) as well. In such cases it varies between having the kanji as the main word with the katakana above it or vice versa.
- To give an alternative word (generally of the same, or similar, meaning)
For example having the furigana びしょう above the word 微笑む or the furigana わけ above the kanji 理由 (in the latter case it would be unclear whether it would actually be said りゆう or わけ - either is possible).