Apple Watch models are divided into three “collections”; Apple Watch Sport, Apple Watch, and Apple Watch Edition. They are differentiated primarily by the materials each use; Sport features aluminum, Apple Watch uses stainless steel, and Apple Watch Edition uses 18-karat gold. Each model has versions with 38 and 42 mm bodies; the 42 mm models have a slightly larger screen and battery. Each model has various color and band options.
From Apple Watch from Wikipedia