When I was a kid my dad had an old mandolin that would get played every now and then. It eventually got broken, but an attachment to the instrument has stuck with me all these years.
Obviously a mandolin is not a guitar. It has a unique sound of its own because, like a 12 string guitar, each course has two identical (and identically tuned) strings. It is tuned like a fiddle, and these days is used mostly (it seems) by bluegrass bands.
Here's a good article about the difficulty of tuning a mandolin.