Pokémon GO for Apple Watch was announced and demonstrated during Apple’s Keynote event, and there's no doubt in my mind that it was most likely done to distract from and mitigate the negative reactions to the AirPods, but that's another story.
Based on the demo, the Apple Watch version of the app will function much like on the iPhone except for the key difference that it will not allow you to catch Pokémon, for that you will still have to use your phone, and perhaps the absence of this feature is not so much an oversight or a lack of capacity issue but more of a built-in incentive to have players purchase the Pokémon GO Pro which launches mid September. You will, however, be able to see nearby Pokémon, collect items from PokéStops, check your player level and egg progress, calories burned and distance traveled during walks. At the end of your travels or workout the app will provide you a summary of everything collected and calories expended, essentially turning it into a fitness app.
According to Niantic, Pokémon GO for Apple Watch and the Buddy update will be available some time this Fall.
Watch the demo presentation by John Hanke below: