As you can see, the biggest change from the original Gear S is the new round display. The watch also seems to feature a premium metal build and sports a very different user-interface. It still runs on Tizen though, so no Android Wear here. However, Samsung is expected to unveil a round smartwatch with Android Wear as well, hopefully alongside the Gear S2.
Samsung seems to have focused heavily on the design aspect with the Gear S2 as the smartwatch appears quite fashionable and not something you would be hesitant to wear in public.
Samsung has promised that it will be revealing the Gear S2 smartwatch at its IFA event this year.