Just to expand a bit on what Andrew said, there are now three "series" of Apple Watch:
1) Apple Watch (the original; sometimes called the first generation, or "series 0," although none of these are marked on the Watch). Came in aluminum and stainless steel (silver and black). Apple Watch Edition was gold.
2) Series 1 (second generation internals). Comes in aluminum only in four colors (space grey, silver, gold, and rose gold). "Series 1" is marked on the underside of the Watch.
To be real clear the Series 1 is First Generation internals WITH a replaced SIP CPU (W1P) to a dual core processor. The processing power is at the same level as the Series 2 BUT everything else is First Generation.
3) Series 2 (second generation internals; also supports GPS on-watch and is waterproof ("water resistant to 50 meters" by Apple marketing)). Comes in aluminum in four colors (space grey, silver, gold, and rose gold) and stainless steel (silver and black). Apple Watch Edition is ceramic. "Series 2" is marked on the underside of the Watch.
What becomes confusing is differentiating a first-generation Watch from a Series 1 Watch, as I've seen many sellers and others refer to a first-generation Watch as "Series 1" when "Series 1" is actually a second-generation Watch and is of the same product generation as "Series 2."