Kefalotiri is a hard Greek cheese usually made from sheep's or goat's milk. In many respects it is similar to kasseri, but it has a strong personality of its own.
Kefalotiri is generally a pale yellow color, almost the color of fresh butter. It is quite firm, but not quite as hard as romano. Kefalotiri is somewhat crumbly due to a distinctive network of crevices and fissures that permeate the cheese.
Kefalotiri is notable for its sharp, piquant flavor, almost as sharp as romano, but with a round whole-milk freshness that is similar to kasseri. It is quite suprising that kefalotiri develops its sharpness in a relatively short aging period of just a few months.
Kefalotiri makes a wonderful breakfast cheese, ideally served with fruit or preserves to help mitigate the sharpness and refresh the palate. It is excellent in pastitsio and in savory pastries. In my opinion it is the cheese of choice to use in saganaki. You can use kefalotiri anywhere you would use another hard, aged Greek cheese, especially if you want to give that component of the flavor the lift of the cheese's sharpness.
As with most "unusual" Greek cheeses, it is best to look for kefalotiri at a Middle Eastern Import Food Store or a cheese shop (or if possible in an ethnically Greek nation). I have never come across any variety of kefalotiri in a supermarket, even one such as Whole Foods with an admirable selection of cheeses.