From http://en.wikipedia.org/wiki/Bass_guitar
[edit] Strings and tuning
The standard design for the electric bass has four strings, tuned E, A, D and G, with the fundamental frequency of the E string set at about 41 Hz, making the tuning of all four strings the same as that of the double bass. This tuning is also the same as the standard tuning on the lower four strings on a 6-string guitar, only an octave lower. String types include all-metal strings (roundwound, flatwound, groundwound, or halfwound), metal strings with different coverings, such as tapewound and plastic-coatings, and non-metal strings made of nylon. The variety of materials used in the strings gives bass players a range of tonal options.
Early basses used flatwound strings with a smooth surface, which had a smooth, damped sound reminiscent of a double-bass. In the 1960s and 1970s, roundwound bass strings similar to guitar strings became popular. Roundwounds have a brighter timbre with greater sustain than flatwounds. Flatwounds are still used by bassists who want a more 'vintage', 'smooth', or 'damped' sound.
A number of other tuning options and bass types have been used to extend the range of the instrument. The most common are:
Note positions on a right-handed 4-string bass in standard EADG tuning.Four strings with alternate tunings to obtain an extended lower range.[1]
Five strings (usually B-E-A-D-G, but sometimes E-A-D-G-C). The 5-string bass with a low "B" provides added lower range, as compared with the 4-string bass. As well, it gives a player easier access to low notes when playing in the higher positions.
A five-string electric bass guitar being played by metal band Metallica's Robert Trujillo.Six strings (usually B-E-A-D-G-C, but sometimes E-A-D-G-B-E or F#-B-E-A-D-G). The 6-string bass is a 4-string bass with an additional low "B" string and a high "C" string. While much less common than 4- or 5-string basses, they are still used in Latin, jazz, and several other genres. A few players have tuned the high C down to a B (giving B-E-A-D-G-B) matching the E-A-D-G-B found on the first five strings of an acoustic or electric guitar.
Detuners, such as the Hipshot, are mechanical devices operated by the right-hand thumb that allow one or more strings to be quickly detuned to a pre-set lower pitch. Hipshots are typically used to drop the "E"-string down to "D" on a four string bass.[2]