One Tag to rule them all, One Tag to find them, One Tag to bring them all and in the darkness bind them
Canonical questions are those that have the many arms of duplicates pointing back to it as the one truest presentation (or near enough) of the problem. In other words, the most cited when duplicate questions are found.
Canonical answers are those posted to a question that serve to cower all others in their breadth, research and utter tremendous weight of reason and accuracy.
These two post types often find themselves on the same page.