The iPhone 7 is now fully water-resistant(it can take a shallow dunking).
The camera takes notably better photos, especially in low light, and adds the optical image stabilization feature previously restricted to the 5.5-inch Plus model.
The battery lasts a little bit longer - sometimes it’s notable, other times it feels just slightly better than the iPhone 6S.
The processor is definitely faster, although keep in mind that last year’s iPhone 6S has more than enough speed for most people’s needs.