Literally, kunoichi means 'woman',based on a baroque Japanese-language visual pun on the kanji 女-'Onna'("Woman,Female"), and how it can be written with first the hiragana character く-'Ku', then the katakana character ノ-'No', and finally the kanji for the number one, 一-'Ichi.'
According to the tangle of legend, folklore, and actual history about ninja, the kunoichi were trained for more subtle tasks than male ninja, including espionage, reconnisance, messenger work, and sudden hit-and-fade attacks and assassinations. Inevitably, their duties also included the seduction of high-ranking enemies when ordered, and their training included manipulating their (usually male) opponents into dropping their guards before attacking. In addition, because of the clan focus of ninja society, the kunoichi were expected to be able to defend their homes when it was necessary, and trained to fight accordingly.
Many of their weapons were designed to be easy to conceal, including common feminine objects (e.g. combs, folding fans) modified to serve as weaponry when the time was right. They also were trained in more conventional weaponry (swords, bows and arrows) for use in open conflict. However, even more than usual for ninja, kunoichi valued deception, so they tended to prefer surprise attacks followed by quick retreats to drawn-out 'open' battles.
Kunoichi are popular character types in popular entertainment from, or just involving, Japan. Video games, especially, seem to have them at every turn - for instance: Yuffie and Rikku from the Final Fantasy RPGs, Ibuki from Street Fighter 3, Kasumi and Ayane from the Dead or Alive fighting game series, Taki from Soul Calibur, and, of course, SNK's infamous Mai Shiranui. The historical accuracy of their fighting techniques, personalities, and outfits is left as an exercise for the reader.
Sources used for this writeup:
No formal research went into this writeup. Please add any information you can.