DVDs for North America are Code 1
DVDs for Europe are Code 2
DVDs for Australia are Code 4
if you have a CODE FREE DVD player you can watch all
there are also some uncoded DVDs out there - but most commercially available DVDs are coded
IMPORTANT: some DVD players (and especially those on PCs & Laptops) come unprogrammed and you can switch from one to the other - BUT: it only lets you switch 2 or 3 times and then you are STUCK ON THAT CODE FOREVER !!!!
unless you or a good friend is really good at doing stuff to PCs that most of us can't
here a link for wikipedia explaining the various codes and DVD player types and much much more...
and yes - most DVD players can be adjusted so they play any code - however - this can generally only be done by someone who knows HOW - and some (or probably most) of us unfortunately don't
http://en.wikipedia.org/wiki/DVD_region_code
i've been battling with the various region differences since i moved to switzerland in the mid eighties... first VHS and now DVD