The biwa is a Japanese plucked lute chordophone with four strings. Each string is extremely close together near the bridge, but expands further apart down the pear shaped body. A particularly notable feature is the neck and tuning pegs are bent 90 degrees backward. Therefore, if the biwa is held upright, the bridge and neck are almost parallel to the ground. The strings are plucked with a plectrum. These plectrums are similar to guitar picks, but shaped more like "T"s and are about as big as a hand.
Entering Japan around the 8th century, it is a direct descendant of the Chinese pipa. The pipa is another plucked lute chordophone with many of the same properties which came from a Middle Eastern plucked lute chordophone, the tar. Therefore, the biwa, like many instruments, has historical roots back in the cradle of civilization. Some styles of playing the biwa are closely related to the shamisen, another Japanese lute that more closely resembles Western lutes and guitars.
It can commonly be found in gagaku ensembles, which is essentially classical Japanese music and arguably the oldest form of orchestration in history. It isn't considered to be a solo instrument in modern times, although some entertainers have used it to accompany story telling. In the past, blind musicians played the biwa solo, but much of that style has been lost.