What we call pizza today, and is the type most commonly consumed in the US is based on a traditional authentic Neapolitan Pizza, the Pizza Margherita.
Pizza Margherita is made with tomato, sliced mozzarella, basil and extra virgin olive oil. There are specific rules about making the dough and the baking of the pizza in order for it to be considered a "true" traditional Neapolitan pizza (there are only two main variations of pizza considered to be authentic).
The dough must be made with Neapolitan wheat flour and baked for 60 to 90 seconds in a wood-fired brick oven at 485°C. The pizza must not exceed 35 cm in diameter or be more than a third of a cm thick at the center. The toppings must be locally produced.
The other authentic Neapolitan pizza is the Pizza Marinara. The toppings include tomato, oregano, garlic, extra virgin olive oil and usually basil (basil was not an original ingredient, however). There is no cheese on the Pizza Marinara, something that might make it much less popular as an everyday pizza choice in the US, but this pizza is made in many pizzerias in Naples and may be enjoyed by tourists looking to sample authentic, traditional pizza.
If you would like to try baking this traditional cheese-less pizza at home, there is an excellent recipe at e*rcps.com