A guitar that has 12 strings, 3 bass strings, 3 tenor strings, and 6 octave strings that pair with each string. Each octave string is tuned to the same key as it's other string, but also remain an octave higher. They create a rich and powerful sound that pairs good with loud and forceful singing, and is commonly used by buskers and professional performers. All tunings of course work with these, but the common and most effective tunings are standard tuning, and are commonly tuned to key of D, C, and B, tuning.
A famous and very talented performer, Lead Belly, used a 12-string acoustic guitar as his signature instrument, and usually had his guitar tuned to Standard B tuning for a wide range of songs and notes. He one of the most influential guitar players, and introduced many people to the 12-string.