As has been pointed out, the strings are six sets of two. The lower strings have a "partner" one octave above the normal tuning, the higher strings are the same pitch. The end effect is that you get sort of a natural chorus effect.
Chords and stuff are played just like you'd play a six string, but it takes a bit more finger strength and coordination to get it right (particularly if you are picking and not strumming). Neck is a tad wider to accomodate the extra strings as well.
They are a pain in the ass to tune, so I'd recommend a clip-on tuner if you don't already have one. I put off getting one forever as a point of pride, but now that I have one I don't know wtf I didn't get one years ago.