I didn't grow up in Chicago but in my experience visiting relatives in the southwest suburbs the thin-crust pizza I ate there (usually Aurelio's) was pretty distinct from other kinds I've eaten in the Great Lakes states. The description other commenters have given of a "cracker" crust is apt. A pizza parlor where I worked in Michigan, for example, had thin pies cut "party-style" but with a much chewier crust.