I kind of always knew it was ham...but ever since I saw it at a Trader Joe's...I wondered. So, from Wikipedia:

Prosciutto (English pronunciation: /prəˈʃuːtoʊ/,[1]) is the Italian word for ham. In English, prosciutto almost always used for an aged, dry-cured, spiced Italian ham that is usually sliced thin and served uncooked; this is called prosciutto crudo 'raw ham' in Italian and distinguished from prosciutto cotto 'cooked ham'. The most renowned and expensive legs of "prosciutto" come from central and northern Italy (Tuscany and Emilia in particular), such as Prosciutto di Parma, and those of Friuli-Venezia Giulia, such as Prosciutto di San Daniele.