Gruyère is the finest and best-known type of Swiss cheese. It has been made since the 12th Century, when cheese was used as currency in Switzerland. It is cream-colored and firm, but softer and smoother than the next-best-known Swiss cheese, Emmental, and has smaller and infrequent "eyes" (holes) - about the size of peas.
The production and naming of the real-McCoy Swiss Gruyère is strictly controlled (as with Champagne in France and Parmigiano-Reggiano cheese in Italy), and the wheels, which can weigh up to 77 pounds, have the word "Switzerland" stamped all over them. But there are many imitations and descendants of this cheese. Beaufort and Comté (or Gruyère de Comté), from France, and Appenzell, from Switzerland, are Gruyère-type cheeses, and there are versions made in the US.
Gruyère is one of the world's great eating cheese and also a great melter.