The banjo could architecturally be considered as a type of lute or guitar, although its origin is most likely African rather than European or Asian. It seems to have developed from the African kora, which is guitar-like, but with the soundbox not made from wood, but a gourd with skin stretched over it, in similar fashion as a vellum is strecthed over a drum pipe. Traditionally the neck was made from materials such as a stick. Originally gut was used for strings, as used to be the case for most string instruments in the past. There are many variations on this basic banjo design. The soundbox of modern versions are made from a rim (wood, or artifical materials) instead of a gourd, and artifical vellum instead of animal skin.
Its unique sound is created by the different types of material used for the soundbox, which is also much rounder than other string soundboxes such as the violin or guitar, and thus create different harmonics.
Source: http://www.miayf.org/string/plucked-string-instruments/banjo/
http://www.youtube.com/watch?v=1jn3KCZEqxc