Basically:

  <div class="row">
    <div class="span6">
       Hello
    </div>
    <div class="span6">
       Uncentered World
    </div>
  </div>

and compare it with:

  <center>
    Centered
  </center>

Note there are extra 20px at the end.

Resizing the window makes everything drag to the left. So much for having a grid.