A
Japanese guitar-like instrument, a small box (covered with cat or dog skin -- ooh yukky!), a long neck which can be dismantled for transportation*, and three
strings. These can be variously tuned: in a
kabuki play, solemn music or
hon-choshi is done in B-E-B', cheerful music or
ni-agari is done in B-F#-B', and serene or melancholy music or
san-sagari is in B-E-A.
The samisen is also the principal instrument in bunraku or puppet drama. It was introduced from the Ryukyu Islands in the sixteenth century. It is played with a bachi, a large plectrum with a percussive effect. A geisha is expected to be able to accompany herself on the samisen.
A very taut version of the instrument from Hokkaido is known as the Tsugaru samisen; it is covered in dog skin.
The plectrum can be made of tortoiseshell, the box from koki (Indian barwood), and other bits are ivory, so more or less the entire thing bar the skin is now illegal to make because it's all endangered species.
It is usually known in the West as samisen, the Kyoto form of the name: in Tokyo it is shamisen. It comes from the Okinawan jamisen or sanshin, which has a more oval box and is covered in snakeskin. This derives from the Chinese sanxian.
* This doesn't mean you can reassemble the pieces into a form of transportation, like a pogo stick or one of these new-fangled mini-scooters, though no doubt some canny Japanese inventor is sanding and polishing one such device even as we speak; it just means you can disassemble it to transport it. Sorry about the ambiguity.