The twelve string guitar is, as its name suggests, a guitar with 12 strings which produces a rich, resonant tone that is sweeter and more harmonious than the standard six string variety. It presents a natural chorus effect due to minute tuning differentials and the subtle difference in string timbre.
Playing a 12 string is a little more difficult than a 6 string due to the string spacing which makes it harder to pick and bend, but the lush, harmonic sound achieved on a 12 string makes the effort worthwhile.
When tuning a 12 string the bass strings are usually tuned an octave apart and the other strings tuned in unison. Most of the major manufacturers have a 12 string acoustic in their range and they can be from the budget model to a high end model such as those by Takamine or Martin.