Since pizza is essentially just a flat crust covered with "stuff you like" the "stuff" is going to vary depending on the location.
Italian pizza tends to be very simple - very little sauce, or even just a crushed tomato sort of smeared on the bread, fresh mozzarella an some basil - that's basically it. Originally it was a dish to use up leftovers, so whatever the chef had lying around was thrown on the crust, and baked.
American pizza comes in two basic varieties - the New York style, which keeps the same thin crust as the Italian, but uses more sauce, more cheese, and adds meat, vegetables, whatever. Then there's the Chicago style which is more like stuffed bread pie than anything else. In America, anything is fair game for being put on a pizza. There are now Korean restaurants that serve pizza with kim-chee on it, for instance. (it's quite good if you like spicy kim-chee!)
Japanese pizzas are based on the American pizza chains, but change out some ingredients based on local tastes. So, at the local Domino's in Tokyo, you can get corn on your pizza - it's a very popular topping. Then there's the seafood pizza which doesn't use any sauce, and is just covered with various bits of fish, squid and octopus. The fresh pizza is then sprinkled with shredded seaweed and drizzled with a Japanese style dipping sauce. If you've ever eaten Takoyaki, it's essentially like that - just in pizza form.