I clicked on
http://www.giant-bicycles.com/en-us/...logy/aluxx/56/
Butting refers to tubing with varying thicknessess throughout its length. There is more material on the ends (anyone remember moron tuning lol?) and the centers are thinner. Single, double, or triple refers to how many "steps" there are in the thickness. Triple butting renders the thinnest, hence lightest tubes.