Aby
they are used just like 6 string guitars although sometimes are used more for spanish style music they are much harder to play

sid
creating or playing music

Computer Guy
There are 2 strings (in different octaves) for each note. It is played the same way a 6-string is, but fingering can be a bit stiff since the strings are doubled. You can play all the same songs. Grandpa

Cole
They use them rather than 6 string because they have a much more interesting sounding tone, some songs that are played on 12 string would be songs like: Stairway to Heaven, Hotel California, and many more.

N
They give you a more stringy tone. They are used according to the artist's preference.

The_Dude
They can be used for any genre of music and are played the same way as a standard 6-string. They produce a unique rich sound that appeal to some artists. They are usually used by rock, blues and folk acts. Here's the great Stevie Ray Vaughn playing his song 'Rude Mood' with a 12-string acoustic: How to tune one:

Average Mike
To clear up some misinformation, the 12 string guitar has six paired courses. The top two courses are tuned in unison and the bottom four courses are tuned in octaves. It can be tuned up to standard pitch (e-B-G-D-A-E) although many players tune it a whole tone or more lower. It provides a fuller sound since there are 12 strings, one that sounds especially good as an accompaniment to vocals. It is difficult to use as a solo instrument since the highest pitch string is the octave G string, not the high E string, so solo runs appear to drop in pitch as you play from the bass to the treble strings.