Hard cheese made with skim milk.

Its protein content approaches meat, while being easier to digest; the only real Parmigiano bears a burnt dot-matrix imprint that says "Parmigiano Reggiano". It is made in Italy, in the provinces of Parma and Reggio, and a tiny bit of the province of Piacenza.
Parmigiano, in order to deserve its name, must be aged for at least six months: at this point you have Parmigiano giovane which means "young". Progressively older cheese deserves the name vecchio and stravecchio. Generally, young Parmigiano is eaten straight, while older grades are used for grating over food. As Parmigiano ages, it becomes darker in colour, and the taste refines, although never to the point of being sharp.

Parmigiano is made with milk from cows that have been fed on grass; in winter grass is not available, and the cows are fed hay. This changes the taste of the milk, which in turn produces a different cheese. The resulting cheese is normally labelled vernengo, which means "pertaining to the winter" in the local dialect.

a Trick: real Parmigiano can be recognized by the fact that, in the mass, there are tiny white spots that look like salt, and in fact are precipitated calcium salts.
Notice also that Parmigiano does not make threads when it melts, and it melts at high temperature.

Imitations: there is a sort of approximately cheese-like dust made by Kraft and sold in cylinders. Avoid. There is also a Parmigiano imitation made in South America, usally called Regianito, which is not bad as cheese goes but has nothing to do with the real thing.
Cheese produced using more or less the Parmigiano technique in Northern Italy is normally called "grana" or "grana padano". It can be quite good.
"Lodigiano", made in the countryside around Lodi is quite tasty - but it is a different cheese.

See also: Parma, parmesan, cheese, gnocchi, pasta