Originally from "banjar," an African string instrument.
The banjo comes in a variety of different forms, including four-string (or plectrum) and five-string versions. In all of its forms it is a poorly sustaining instrument and its playing is characterised by a fast strumming or arpeggiated right hand.
The banjo can be played in several styles and is used in various forms of music. In bluegrass music, which uses the five-string banjo extensively, it is often played in Scruggs style. American old-time music also typically uses the 5-string banjo, but it is played in different styles, notably claw-hammer or frailing. Another characteristic of old-time banjo styles is the use of a wide range of different tunings.
The plectrum banjo evolved out of the 5-string banjo to cater for styles of music involving strummed chords. A further development is the tenor banjo, which also has four strings and is typically played with a plectrum. It is usually tuned CGDA, like a viola, or GDAE, like a violin (but an octave lower), and has become quite a standard instrument for Irish traditional music.