Ricotta salata cheese is originally from Sicily, although it's made in other regions in Italy too. Like regular ricotta, ricotta salata is made from whey rather than curds. Unlike ricotta, however, ricotta salata is pressed and aged for at least 3 months, resulting in a much drier cheese. It has a firm texture and salty flavor (salata means salted), making it ideal for grating. It's wonderful grated over salads or sprinkled on top of risotto or tomato-based pasta.
Some people think ricotta salata is a good substitute for feta, and that's probably because both cheeses are traditionally make with sheep's milk, and both cheeses are salty. But in fact, the two are quite different from one another. The key difference is that feta is aged in brine, giving it a much wetter, more crumbly texture than ricotta salata.
In a pinch, you can use feta cheese instead, but be sure to soak it for an hour in cold water before you grate it.