In USPSA, the division you shoot in is almost entirely set by the gun you bring. Production guns are the most common, so the competition is fierce. Limited guns remove the 10 rd restriction, and can have external modifications like mag-wells, and slide cuts. The Open division is an "anything goes" division that allows optics, crazy holsters, and compensated / ported barrels. Single-Stack is for 1911 lovers and Revolver should be self-explanatory.