Bacon can come from any part of the pig. But most of the time its from the back, sides, or belly. Gammon (bacon) is from the hind leg, and hock bacon is from above the hind trotter (foot).
Actually, Canadian bacon is ham - I lived in Canada for several years as a small child, and that's what all Canadians call it - and it is. Bacon is cured and smoked meat from the rib cage of the hog....
The word 'bacon' is derived from the Old High German word 'bacho' which meant smoked and salted meat from the back or sides of a hog. This is why they call bacon bacon.