The primary difference between deep-dish pizza and most other forms of pizza is that, as the name suggests, the crust is very deep, creating a very thick pizza that resembles a pie more than a flatbread. Although the entire pizza is very thick, in traditional Chicago-style deep-dish pizzas, the crust is thin to medium.
Deep-dish pizza is baked in a round, steel pan that is more similar to a cake or pie pan than a typical pizza pan. The pan is oiled to allow easy removal and create a fried effect on the outside of the crust. In addition to ordinary wheat flour, the pizza dough may contain cornmeal, semolina, or food coloring, giving the crust a distinctly yellowish tone. The dough is pressed up onto the sides of the pan, forming a bowl for a very thick layer of toppings.
Resource. Wikipedia, Deep-dish pizza