The banjo as we know it today is descended from designs that arrived in America by way of African slaves. Although it's associated with country and bluegrass music, the banjo has become increasingly popular in other forms of music, including modern jazz and rock. The distinctive twang of the banjo results from a combination of drum size, bridge, general construction and, most important, its strings, which vary in number depending on the type of banjo.
The Four-String Banjo
All banjos have the first four strings. They deepen in tone from bottom (closest to the ground when the banjo is held) to top: D-B-G-D. The bottom D (usually written as "d") string is the thinnest and the highest pitched of the four strings, and the top D is the thickest and deepest sounding. These four notes do not cover the entire musical scale attainable by a four-string banjo. Manipulating the strings on the frets (thin metal lines under the strings) near the neck of the instrument allows these four notes to become all the notes of the musical scale, extending into several octaves.
The Fifth String
Nearly as common in America as the four-string banjo is the five-string banjo. The fifth string, a G-string, does not extend all the way up the neck of the banjo as the other strings do, and though it is below the deepest sounding string, does not fit the descending scale of sound as do D-B-G-D. This string reaches from the bridge only to the fifth fret of the neck, and has its own tuning dial affixed there. The sound of the string is high and discordant, and is a sound closely identified with bluegrass and "old-time" music (as in the soundtrack to "O Brother Where Art Thou?"). There are many differences between the four-string and five-string banjos. They are considered to be completely different instruments used for completely different music.
The Six-String Banjo
One of the rarer types of banjos that still enjoys a passionate following is the six-string banjo. The six-string banjo goes by many other names, including guitanjo, guitjo, ganjo, banjitar, or bantar. Most of these names indicate the unique property that the sixth string gives a banjo. The sixth string creates something akin to a marriage between a banjo and a guitar. Having the neck of the guitar and the drum of a banjo, it sounds like a banjo but is played like a guitar, with the strings E-A-D-G-B-E, instead of standard the standard banjo strings D-B-G-D.