Peat moss is a soil amendment that is harvested from peat bogs. Many of these bogs are found in wetland areas of Canada. Peat bogs are formed, over the course of very long periods of time, as organic materials partially decompose. Because of the water in these bogs, anaerobic conditions are maintained, conditions that retard full decomposition. The result is that the peat moss is preserved almost indefinitely.
Although sometimes referred to as "peat" for short, peat moss and peat are not, technically, the same thing. "Peat" is the broader term: Peat moss is just one of the products harvested from peat bogs. Other kinds of peat come from the partial decomposition of other organic materials. The "moss" in "peat moss" refers specifically to sphagnum moss (for example, Sphagnum cymbifolium).