The Bouzouki in its Greek form (where it originated) is tuned as the top four courses of a 12-string guitar - DGBE - with octaves on the D and G courses.

Whilst this makes it instantly accessible for a guitarist, I find it of little practical use - you might just as well use a 12-string, because it sounds better!

Some people use cittern-style tunings such as DADA, DGDG or CGCG, which have a lot of advantages in the 'full' sounding chords you can play along with a melody, but you also need a capo to be effective in different keys as suggested above. These tunings use unison tuned course rather than the Greek octaves.

Ain't life complicated?!