Nashville Tuning (aka "High Strung tuning") is a way of giving a 12-string guitar sound from a regular 6-string guitar. Nashville used the same basic notes (EADGBE) but with thinner strings to give the same notes in different octaves. If you took off the regular strings from a 12-string, you'd be left with a Nashville Tuned guitar!

It is often used in recording played alongside another guitar with regular tuning, giving 12-string guitar sound when no 12-string is available. It also has teh advantage that the high and low parts of teh 12-string sound can be setparated in the stereo mix, giving a more spacial sound.

String notes for Nashville Tuning:

1 - E (as per normal tuning)
2 - B (as per normal tuning)
3 - G (one octave higher than normal tuning)
4 - D (one octave higher than normal tuning)
5 - A (one octave higher than normal tuning)
6 - E (one octave higher than normal tuning)

Famous songs that use Nashville Tuning include:
Hey You - Pink Floyd
Wild Horses - Rolling Stones
Jumpin' Jack Flash - Rolling Stones
Comfortably Numb - Pink Floyd
Bye Bye love - Everly Brothers