Bacon is pork belly that has been cured and (usually) smoked.  Pork fatback or loin can also be treated in the same manner, the latter of which is referred to as back or Canadian bacon.  The Italian preparation of Pancetta shares many similarities - it is also produced by curing pork belly, but spices and aromatics are often added to the curing salt, and it is not smoked.

Used as a base in sauces.

Beef Bacon