Lower tuning is different from alternate tunings in that instead of arranging the notes to be played "open" (popular amongst folk and blues guitarists) the strings still follow the standard EADGBE tuning but dropped to a lower interval. Thus a EADGBE tuning becomes DGCFAD, for example. This way a guitarist can achieve a different sound without having to deviate from standard chord positions of the fingers. This method of tuning is popular because it...
- can better accommodate a singer's vocal range
- allows for a deeper and thus heavier, thicker sound to the guitar
- gets better action on the strings, useful for more dexterous solos with pitch bending or hammer-ons
- contributes to the guitar staying in tune more easily
Fender came out with a guitar in the sixties called the Bass VI, which is tuned an octave lower than standard E. This is a strange mixed-breed that looks like a guitar (same design as the Fender Jaguar) but who's notes sound like a bass. John Lennon and George Harrison played this guitar for a time in the late sixties (featured on Hey Jude, Let it Be, Helter Skelter amongst others).
I find tuning to low D to sound especially good on an acoustic guitar. The notes just sound fatter and I've employed this tuning method on a few of my own songs. However, I don't do it often since I don't like the idea of constantly having to re-tune my guitar (I suppose if I had many guitars it would be easier).