On a 12 string, you have strings doubled up. So you have two E's, two A's, two D's, two G's, two B's, and two E's. The additional string is a very light gauge string thats one octave higher.
When you fret a note, you actually are fretting two strings at once. Since one string is one octave higher, the sound is "fuller".
The downside with a 12 string is that it's harder to play and pretty much impossible to bend notes.