The 12 string guitar is exactly as it sounds - a guitar with 12 strings which produces a rich, resonant tone. The 12 strings provide a natural chorus effect due to minute tuning differentials between the strings and a subtle difference in string timbre.
Playing a 12 string can be a little more difficult than a standard 6 string guitar, due to the string spacing which makes it harder to pick and bend, so it is rarely used as a lead instrument and far better suited to the rhythm player.
When tuning a 12 string, the bass strings are usually tuned an octave apart and the other strings in unison. There are limited choices of 12 string electric guitars on the market, but you will find a greater choice of 12 string acoustic guitars. A couple of manufacturers offer a twin neck guitar that includes the choice of both six and 12 strings on one guitar; the most notable piece of music played using this configuration being "Stairway to Heaven" by Led Zeppelin.