Board Thread:Questions and Answers/@comment-64.233.173.46-20160505164127/@comment-6697950-20160505165239

Katakana is the Japanese syllabary usually used for words of foreign origin (English in this case). Furigana are the small ruby characters written either above (in horizontal writing) or to the right of (in vertical writing) Japanese text and is used to explain how to read the word that it accompanies. Furigana text is usually written using either hiragana or katakana characters.

In the original Japanese text of the SAO light novels, almost all skill names are written in katakana (because the names are based on English words), except a few skills that actually are actually named using Japanese words. Such words usually have furigana to explain how the word should be read.

For example, the Sword Skill Sonic Leap was written as ソニック・リープ, while on the other hand, Tsujikaze was written in the original text as. Notice the small characters above. That's what furigana is.