Due to the wide influence of Italian and Greek immigrants in American culture, the United States has developed quite a large number of regional forms of pizza, many bearing only a casual resemblance to the Italian original. During the latter half of the 20th century, pizza in the United States became an iconic dish of considerable popularity, and may have contributed to the decline of the British pie heritage previously common in American cuisine.

• New York-style pizza is a style originally developed in New York City, where pizza is often sold in oversized, thin and flexible slices. It is traditionally hand-tossed, moderate on sauce, and moderately covered with cheese essentially amounting to a much larger version of the Neapolitan style. The slices are sometimes eaten folded in half, or even stacked, as its size and flexibility may otherwise make it unwieldy to eat by hand. This style of pizza tends to dominate the Northeastern states and is very similar to the basic style common through the United States and known simply as pizza. It is often referred to as pan-style pizza, but note that Pizza Hut's "Pan Pizza" is a thick-crust variety. Many pizza establishments in the New York metropolitan area offer two varieties of pizza: "Neapolitan", or "regular", made with a relatively thin, circular crust and served in wedge-shaped slices, and "Sicilian", or "square", made with a thicker, rectangular crust and served in large, rectangular slices.
• Chicago-style pizza, or Chicago-style deep dish pizza, contains a crust which is formed up the sides of a deep-dish pan. It reverses the order of ingredients, using crust, cheese, filling, then sauce on top. Some versions (usually referred to as "stuffed") have two layers of crust with the sauce on top. Deep-dish pizza was purportedly invented and first served in 1943 at Pizzeria Uno, which is still operating along with its twin restaurant, Pizzeria Due, in the River North neighborhood.
• Chicago-style thin crust pizza has a thinner crust than Chicago-style deep dish, and is baked flat rather than in a deep dish pan. The crust is thin and firm enough to have a noticeable crunch, unlike a New York-style pizza, yet thick enough to be soft and doughy on the top. The crust is invariably topped with a liberal quantity of southern-Italian style tomato sauce, which is usually quite herbal or highly spiced, and typically contains no visible chunks of tomato. Next, a layer of toppings is added, and a layer of mozzarella cheese which frequently separates from the bottom crust due to the quantity of tomato sauce. Chicago-style thin crust pizzas are cut into three- or four-inch squares, also known as "party cut," as opposed to a "pie cut" into wedges. The small size of the squares makes it unnecessary to fold the slices. Chicago-style pizza is prevalent throughout the Midwestern USA. A chain that is well known for its Chicago-style thin crust pizza is Home Run Inn
• St. Louis-style pizza is a variant of Chicago-style thin crust that is popular in and around St. Louis, Missouri. The most notable characteristic of St. Louis-style pizza is the distinctively St. Louisan provel cheese used instead of (or rarely in addition to) the mozzarella common to Chicago-style thin crust. The two largest St. Louis-style Pizza chains are Imo's Pizza and Cecil Whittaker's Pizzeria.
• California-style pizza (often termed in the United States gourmet pizza) refers to pizza with non-traditional ingredients, especially those that use a considerable amount of fresh produce. A Thai-inspired chicken pizza with peanut sauce, bean sprouts, and shaved carrots is a popular variant in California-style pizza restaurants, as are pizzas that use chicken and barbecue sauce as toppings. Duck, spinach, and goat cheese are also used. The California Pizza Kitchen chain of restaurants was influential in popularizing this style. In addition, there are sweet pizzas in existence, which list jam or peanut butter among others as ingredients.
• Greek pizza is a variation popular in New England; its name comes from it being typical of the style of pizzerias owned by Greek immigrants. It has a thicker, chewier crust and is baked in a pan in the pizza oven, instead of directly on the bricks. Plain olive oil is a common part of the topping. Variations in other parts of the country include using feta cheese, Kalamata olives, and Greek herbs such as oregano.
• Hawaiian pizza has ham and pineapple toppings and is especially popular in the Western United States. Ham and pineapple is also a popular topping combination in Australia and Canada, but notably not in Hawaii. This type is also common within the EU as Pizza Hawaii.
• Grilled pizza, invented in Providence, Rhode Island, uses a fairly thin crust cooked on a grill; the toppings are placed on the baked side after the pizza has cooked for a bit and flipped over.
• English muffin or French bread pizza and pizza bagel is a common convenience pizza made at home in an oven or toaster, usually with a simple topping of spaghetti sauce, sliced or shredded cheese, and perhaps pepperoni. French bread pizza is sometimes available commercially as a frozen meal.

This excerpt is courtesy of Wikipedia, for more about pizza and pizza styles, visit http://en.wikipedia.org/wiki/Pizza