What does it do? haha
Seriously, Do you have to like fret 2 strings at once?
And what is the tuning?
How expensive is one?
Is there any famous piece of music played on a 12 string? link pls?

It's just a different tone. The strings are arranged in pairs, so you can fret and play it just like an ordinary guitar. Six strings are tuned like a normal guitar, the ones they're paired with are tuned an octave higher (except the high E and B, those are in unison). They're a little more expensive than a normal guitar. Lots of famous pieces were recorded with them; the middle part of Stairway to Heaven, for instance, or California Dreaming by the Mamas and the Papas.
Wish you were here is the most popular one id say. That intro has a 12 string guitar. They just sound better*. *=subjective of course.

Heres a list I googled in 4 seconds though. I'm not going to verify the content but it gives you an idea.

A Hard Day's Night (The Beatles)
Amanda (Boston)
Anytime At All (The Beatles)
Eight Miles High (The Byrds)
Father Christmas (Greg Lake)
Free Falling (Tom Petty)
Give A Little Bit (Supertramp)
Hotel California (The Eagles)
If I Needed Someone (The Beatles)
Maggie May (Rod Stewart)
More Than A Feeling (Boston)
Mr. Tambourine Man (The Byrds)
Sister Golden Hair (America)
Stairway to Heaven (Led Zeppelin)
Still, You Turn Me On (ELP)
Suite Madame Blue (Styx)
The Rain Song (Led Zeppelin)
The Song Remains the Same (Led Zeppelin)
Ticket To Ride (The Beatles)
Turn! Turn! Turn! (The Byrds)
Wanted Dead or Alive (Bon Jovi)
Wish You Were Here (Pink Floyd)
You Wear It Well (Rod Stewart)
You've Got To Hide Your Love Away (The Beatles)
Gibson Les Paul Traditional - 11
Fender Stratocaster MIM - 04
Fender Mustang - 04
Taylor 214ce - 10
Last edited by teamhex at Mar 24, 2011,