Mozzarella is one of the only cheeses you'll encounter on a regular basis that's made from the milk of water buffalo. In fact, most people who encounter buffalo mozzarella don't even realize that it's made from buffalo milk; they assume it's as closely related to buffalo as Buffalo wings are. But in fact, the highest-quality mozzarella in the world is in fact made from water buffalo milk, and it's a truly superior product to mozzarella made from cow milk.
(It is worth noting that the water buffalo discussed in this article is not the same as the American bison, often referred to as "buffalo" in the United States. If you order a "buffalo burger" from a menu, you'll be eating a different animal than the bufala Mediterranea Italiana, whose milk makes the finest mozzarella.)
Try buffalo milk mozzarella next to the cow milk variety and you'll quickly notice that there's really no comparison between the two. Buffalo mozzarella is creamier, softer, and far more flavorful than the cow-milk stuff, with a tanginess and depth of flavor that's all its own. This can be attributed to water buffalo milk's high fat content - nearly double that of cow's milk - but just like goat's milk tastes different from sheep's milk, cow's milk tastes different from water buffalo milk, and water buffalo milk simply tastes better when turned into mozzarella.
If you want to sample buffalo mozzarella for yourself, it's going to be a little bit more difficult to track down than you might think. Because it's nearly impossible to produce domestically for several reasons (The New York Times has a great article on those reasons here), it's actually "one of the most elusive cheeses on earth," the Times notes. It's primarily produced in the hills around Naples, which is in the Italian region (Campania) where water buffalo are most plentiful, and coaxing the perfect ball of mozzarella from their milk is a task that takes years of training. And even though water buffalo mozzarella can technically be produced anywhere, the officially government-regulated (DOC) Mozzarella di Bufala Campana can only be produced using a traditional recipe in the regions of Campania, Lazio, Apulia, and Molise.
For these reasons, you can only find it in the States at upscale Italian restaurants, specialty shops like Eataly, and high-end cheese shops. Or you can just travel to Naples; if you walk into a cheese shop there and ask for mozzarella, you'll receive a ball of buffalo mozzarella that's probably only hours old. If you want the cheaper cow's milk variety, you have to ask for it by a different name entirely, one that's been popping up on more and more Italian menus these days: fior di latte.