12 String Guitar

The 12 string has a double set of strings, wherein the four thickest strings have a different thickness compared to each other.
The highest 2 strings (B and E string) pairs are the same tuning and the other 4 pairs are each  tuned an octave differently.
The advantage of such a guitar is that they have a lot of volume.
It sounds like there are two players.

12 String guitars have a very strong construction because of the great force that the double set of strings excersises on the neck and the soundbox.
These guitars are therefore heavy.
You also need much strength in your fingers to press the strings. They play heavier than 6-string guitars.
They are widely used for playing chords, but they can also be played differently. Just click the video link.
There you can see the probably most famous song for a 12 string 'Hotel California' by the Eagels.