Kunoichi is the term for a female ninja, or practitioner of Ninpo. The term is thought to derive from the names of characters that resemble the three strokes in the kanji character for woman (女, onna); said in the order they are written: ku (く) - no (ノ) - ichi (一). In Japanese, the 'ku' is written in hirigana, 'no' in katakana and 'ichi' in kanji - add the three together, as in the image below, and you get the Japanese word (in kanji) for woman.
Another proposed derivation is from the character 九能一 (能 "nō" : talent) with Japanese numbers "ku" (九) for "nine", and "ichi" (一) for "one", meaning "nine talents in one person", though no creditable source lists these "nine talents".
The training of the kunoichi was different from the male ninja. Their training focused more on disguise, poisons, and using their gender as an advantage. They were trained in close combat and this knowledge was to their benefit if they were caught. Some of their disguises were as geishas, prostitutes, fortunetellers, servants and so on, in order to get very close to the enemy, or to use this intimacy to obtain information.
Kunoichi could hide their weapons in their disguise. Those weapons were: small blades up their sleeves, or in their belts, even in musical instruments or in sex toys, poisons pins in their hair, or long nails used for raking, blinding powders, ropes and fans could be hidden and used at close range. There were also trained in more psychological warfare and mind manipulation. They could play on the emotions of the enemy with highly trained skills of intuition.
Modern day kunoichi fulfill the same roles as men, playing parts in security, law enforcement, private investigation (where they can still use various disguises and means unique to the gender).
In Bujinkan, the women train and study the same skills and techniques as the men. The emphasis is on effectiveness as power and strength is not a factor. This can be an advantage in learning the effortlessness of a technique, not being able to fall back on muscular strength. In real life situations, the combination of power and strength to the effortlessness of a technique can be formidable.