Greek pizza can refer to either of two styles of pizza.
One usage refers to a pizza with typically (or stereotypically) Greek ingredients as toppings. These include authentic toppings such as feta cheese, onion, Kalamata olives, fresh tomato, and green bell pepper, as well as others borrowed from unrelated Greek dishes such as gyros meat or others incorrectly believed to be typical of Greek cuisine, such as spinach.
The other usage refers to a style of pizza crust rather than its toppings. This style is baked in a pan, instead of directly on the bricks of the pizza oven (as is more traditional for Italian pizza.) It has a thick, chewy crust that is usually quite oily. This style of pizza is referred to as "Greek pizza" even when it has non-Greek toppings, since it is typical of pizzerias owned by Greek immigrants. These establishments often also sell Greek specialities, such as Greek salads and gyros, and tend to brand themselves as "Pizza and Pasta" or as a "House of Pizza"; a code signifying that it is not an Italian restaurant, but a Greek restaurant serving Italian style food.
In the United States, the latter usage appears to be specific to New England cuisine.
Thanks RedGreen. Yes... I was referring to the second description. My home state is Massachusetts, and Greek pizza is MUCH more common there than Italian. Also, I think most there would agree that it makes for a better pizza.