The banjo is a string instrument, derived from "banjar," an African string instrument. Some etymologists derive it from a dialectal pronunciation of "bandore", though recent research suggests that it may come from a Senegambian term for the bamboo stick used for the instrument's neck.

The modern banjo comes in a variety of different forms, including four-string (plectrum and tenor) and five-string versions. In all of its forms the banjo is a poorly sustaining instrument and its playing is characterised by a fast strumming or arpeggiated right hand, although there are in fact many different playing styles.

The banjo consists of a wooden or metal rim with a plastic (MylarŪ) or skin (calf or goat) drumhead stretched across it, a neck mounted on the side of the rim, a tailpiece mounted opposite the neck, four or five strings, and a bridge. The woods used in construction vary, but are often combinations of maple, walnut, and ebony for fingerboards, pegheads, and the tops of bridges. In the five-string banjo, the fifth peg is normally on the side of the neck, although some English versions mount the fifth string tuner on the tuning head with the others, and route the string through a tube in the neck where it exits near the fifth fret.

The earliest banjos were unfretted, like the African and Asian instruments that inspired them, but most banjos today are fretted. Banjo strings are most commonly metal, although nylon and gut used on simple fretless banjos and by players of the classical banjo style. The two most common modern day banjos are the resonator banjos which have a detachable chamber, or resonator, on the back of the rim and the open back banjos which does not have a resonator.

The banjo can be played in several styles and is used in various forms of music. In bluegrass music, which uses the five-string resonator banjo extensively, it is often played in Scruggs style named after Earl Scruggs, melodic or Keith style, or two-finger style also called Reno style named after Don Reno. In these styles the emphasis is on arpeggiated figures played in a continuous eighth-note rhythm.

American Old-time music typically uses the five-string open back banjo, it is played in a number of different styles, the most common of which are called claw-hammer (or "clawhammer") and frailing, both characterised by the use of a downward rather than upward motion when striking the strings with the fingers. Frailing techniques utilize the thumb to catch the fifth string for a drone after each strum, or the thumb is used to pick out additional melody notes in what is known as 'drop-thumb'.

Many tunings are used for the five-string banjo. Probably the most common, certainly in bluegrass, is the open G tuning: gDGBd. In earlier times, the tuning gCGBd was commonly used instead. Other tunings common in Old-time music include double C (gCGCd), sawmill or mountain minor (gDGCd), and open D (f#DF#Ad). These tunings are often taken up a tone, either by tuning up or using a capo.

The fifth (drone) string is the same gauge as the first, but it is five frets shorter (3/4 as long). This presents special problems for using a capo to change the pitch of the instrument. For small changes (e.g. going up or down one or two semitones) it is possible to simply retune the fifth string. Otherwise various devices are available for effectively shortening the string. Many banjo players favour the use of model railroad spikes (usually installed at the seventh fret and sometimes at others), under which the string can be hooked to keep it pressed down on the fret.

The plectrum banjo has four strings and is missing the shorter fifth string; it is usually tuned CGBd. As the name suggests, it is usually played with a guitar-style pick (that is, a single one held between thumb and forefinger), unlike the five-string banjo, which is almost always played with a set of three fingerpicks, or occasionally with bare fingers. The plectrum banjo evolved out of the five-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 has a shorter neck than the other banjos and 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.

A number of hybrid instruments exist, crossing the banjo with other stringed instruments. Most of these use the body of a banjo, often with a resonator, and the neck of the other instrument. Examples include the guitar banjo, the banjo mandolin and the banjo ukulele, or banjolele. These were especially popular in the early decades of the twentieth century, and were probably a result of a desire either to allow players of other instruments to jump on the banjo bandwagon at the height of its popularity or to get the natural amplification benefits of the banjo resonator in an age before electric amplification. Instruments using the five-string banjo neck on a wooden body (for example, that of a bouzouki or resonator guitar) have also been made, though these are not so common.

'Banjo' was also the nickname of Australian poet A.B. Paterson.