Kotos are about 180cm long and have 13 strings that are strung tautly across 13 movable bridges along the length of the instrument. Players make base pitches by moving these bridges before playing, and three finger picks (on thumb, forefinger, and middle finger) are used to pluck the strings.
The Koto was introduced to Japan in the 7th to 8th century from China (the Chinese inspiration was probably the gŭzhēng). It was initially played in the royal court only, but this situation changed in the 17th century -- primarily because of the influence of one man: Yatsuhashi Kengyo (1614-1684).
Yatsuhashi Kengyo was a blind shamisen player who learnt koto from an "official" court player named Hosui, in defiance of the rules which then stated that koto could not be taught to blind people (or women, incidentaly). Possibly because of his personal experience with these restrictions, Yatsuhashi spent the rest of his life making the koto more accessible.
He invented a new "plain tuning" (hira joushi in Japanese) to play the common peoples' songs more naturally. He composed (or is credited with composing) songs that are still irreplacable staples of the koto repertoire today, including Rokudan and Midare. (These compositions were partly responsible for the koto becoming respected as a solo instrument in its own right.) Perhaps most importantly, his example led other non-elite, including women, to learn the koto too.
Since the Japanese music scene was made over in Western pop music's image, the koto has become less prominent (although many well-to-do young women learn the instrument to help develop an aura of "refinement" that will theoretically attract a better class of husband). However, it is still developing as an instrument; works are written for and performed on 20-stringed and bass kotos, and a new generation of young players like Yagi Michiyo are finding places for the koto in today's jazz, pop and even experimental music.