A clan from Guam, StH stands for "Support the Habit."
They play various games including the map variant DotA of WC3, Call of Duty 4, and Gears of War, then occasionally Halo 3.
Members of this clan add the clan's name to the beginning of their gamertag.
"Holy hell, StH.Axial just screwed me over. And again on DotA, then on COD4, once more on Gears. Did I mention Halo 3?"