Real mozzarella, the indispensable ingredient for pizza, lasagna, insalata caprese and so many other dishes, is made from the milk of water buffalo (bufala), mostly in Campania and Lazio.
The name comes from mozzare, 'to cut': the cheese is stretched and kneaded (pasta filata); when it's tender and soft, the cheese maker lops off bits to form each individual cheese. You can also find braided mozzarella (treccia). Fresh mozzarella has a very high moisture content and should be used the day it's made, or kept in brine for up to a week. The commercial kind is drier, and can be kept for much longer.
The similar product made from cow's milk is properly called fiordilatte.
Mozzarella in carozza ('in a carriage') is a Neapolitan standby: sandwiches of white bread sliced diagonally, with a slice of mozzarella inside, dipped in egg and breadcrumbs and fried (recipe here).
Mozzarella nella mortella is wrapped in myrtle leaves, once a way of transporting the cheese to the town, and now prized on its own for the subtle myrtle flavouring of the cheese. In the Ark of Taste.