Despite their prominence in "Italian" style cuisine, tomatoes were not native to Europe until they were brought over from the New World in 1522. At first they were believed to be poisonous, but the lower-classes began to incorporate them into their cuisine over the next several years. The first pizza to use tomatoes originated in Naples. Tomatoes were chopped and added directly into the yeast dough.
Tomatoes are most commonly used in pizza for sauce. In many worldwide pizzerias, sauces are seasoned with olive oil and Italian herbs, but in most traditional pizzas, they can be a sole-ingredient sauce with nothing else added. This mostly depends on the recipe of the pizza being made, which varies from pizzeria to pizzeria.
Some pizzas include sliced tomatoes as a topping, especially the Margherita, which includes tomato slices through the pizza surface. However, it can also be cut on cubes, again, depending on the restaurant style.