The modern banjo is a direct descendant of an instrument whose history goes back to West Africa. A folksy stringed instrument was brought over to America by slaves and its use was spread throughout the South. Ultimately, the banjo became a standard part of minstrel shows, and then its usage worked its way into country and folk music across the nation. These days most banjos have frets, but the originals were entirely fretless. There are two basic styles of banjos: resonator and open back. Resonator banjos have a "resonator" attached to the back of their rim, while open back, much as their name would suggest, have an open backside, and no resonator.
Most banjos today have five strings, but they can be tuned and in fact played in a plethora of styles. Bluegrass musicians use what is called a "Scruggs" style of playing. This playing style is named after famous bluegrass originator Earl Scruggs, and it involves a complex system of fingerpicking. The player uses their thumb and their first two fingers to pick a series of fast paced notes which will ultimately form the melody.
In addition to five stringed banjos, there is also a four string version called a plectrum banjo. This style of banjo is tuned to CGBD, and it is played more like a guitar, i.e. with a pick held between the thumb and forefinger. Banjos are a highly personalized instrument, and their various tunings and styles reflect their extreme diversity and flexibility. They can be found in everything from bluegrass and country bands all the way to traditional Irish folk music. If well taken care of, used banjos can be a real bargain. For the collectors out there you can even find great vintage banjos. For those looking for a new banjo, there are a number of good banjo makers out there. You can even find high quality Gibson banjos that boast the same craftsmanship as their famous guitars.