@discographer-- Technically, mozzarella can be made from any type of milk and there is mozzarella out there made from cow's milk, sheep's milk and goat's milk. It's true that mozzarella made from the milk of water buffalo is most prized though.
I think that the milk of water buffalo has a higher percentage of fat when compare to cow's milk for example. So the mozzarella made from buffalo milk tends to be creamier and tastier. I think this is the most commonly used mozzarella in Italy.
Like the article said, it's not very common in the US. It's only really available at Italian groceries which import buffalo mozzarella. It has to be imported and consumed quickly though because it can go bad rather quickly, especially fresh mozzarella. Our family mostly eats mozzarella made from cow's milk or sheep's milk. I think that sheep's milk mozzarella is an acceptable alternative to buffalo milk mozzarella.