Guanciale is made from pork cheeks (very fat porks apparently). The meat is washed in wine, seasoned and left in a stone niche for 40 days to marinate. After that they hang it to dry and presto - you can eat it. You don't really eat raw guanciale - it is a highly savory piece of white pig fat better used for cooking. You melt it down in a saucepan and use the fat to fry further ingredients who will become infused with this amazing taste. It is not part of any diet I've come across, but even a small quantity will get you the taste, so it's worth trying even if you are dieting.
Many Italians do not know about guanciale. Italy is a large country and regions are very different from one another. People from the North will cook Spaghetti all'amatriciana, a dish from the center, with pancetta and parmigiano. They might even give you a funny look if you mention they should have used guanciale and peccorino.
But this unique bacon is nothing like pancetta. It gives an enormous amount of porky flavour to the sauces and your guests will wonder where it comes from as they hand their plates for more. Every single time I've spoken about guanciale to Italians who knew about it, they tell me how they used it with an immense smile on their faces.
"Oh me, I just cut some thin slices and put it over the salad with croutons and hard boiled eggs," a lady in Bologna told me, "but the best is to cut it in minuscule dices, fry them with no extra fat and them add beaten eggs. The very best omelette you can make."
And so on. You can hit up your "Mama's extra authentic Italian cookbook" all you want, and it will probably not mention guanciale because the fact is, you can't find guanciale outside Italy. And even in Italy, it is widely available only in some regions. Elsewhere people will assume, since you are a tourist, you just mixed up two words. If you're Italian, the would think it's a dialectal name for pancetta. But it's not.