Guanciale is an air dried, dry cured cheek (jowl) of pork. The cheek (Guancia in Italian) is cured using the same sort of cure used for pancetta but is coated in cracked black peppercorns and thyme.
It's really a poor man's version of pancetta so you won't find it in any UK supermarket but (as with all Italian "peasant food") is a great ingredient that speaks for itself and will transform any simple dish like pasta carbonara or al'amatriciana into something authentic and memorable.
Sorry, I'm getting a bit carried away here. If you've used it you'll understand.
I've not seen it for so long, I had a pig's head to use recently and it didn't cross my mind to make Guanciale! DOH!!!!
Gutted!!!!!