1. The A10 Fusion quad-core main processor is up to 2x faster than the A8 processor in the iPhone 6. And it is significantly faster than the A9 processor in the iPhone 6s.
2. The 6-core graphics processor is up to 3x faster than the iPhone 6 and significantly faster than the the iPhone 6s.
3. The M10 motion processor replaces the M9.
4. The Retina HD display now comes with wide color gamut.
5. There's a new Taptic engine.
6. Screen brightness increased to 625 cd/m2 (or 25% brighter than the iPhone 6s).
7. The 450Mbps LTE is 50% faster than the iPhone 6s and 3x faster than the iPhone 6.
8. The camera in the 7 and 7 Plus feature f/1.8 aperture compared to f/2.2 aperture of the 6s, 6s Plus, and SE.
9. The camera lens now has six elements instead of five.
10. The camera has wide color capture for photos.
11. The camera adds body detection added to face detection for both still photos and video.
12. The Quad-LED True Tone flash replaces Dual-LED True Tone.
13. The iPhone 7 and 7 Plus both have optical video image stabilization (Previously it was only available in the 6s Plus model.)
14. The second camera in the iPhone 7 Plus provides optical zoom of 2x for both still images and video. It provides up to 6x digital zoom for video and up to 10x digital zoom for still images.
15. The iPhone 7 and 7 Plus FaceTime camera has been increased to 7-megapixel (versus 5-megapixel for iPhone 6s).
16. FaceTime camera now has wide color capture and live photo support. It also supports auto image stabilization.
17. Battery life has increased for some functions. Internet use on 3G and LTE increases 1 hour to 13. WiFi use increases 3 hours to 16.
18. Size of the 7 is unchanged but it weighs 4 to 5 grams less than the 6s.
19. Stereo speakers replace the single speaker.
20. It is now splash, water, and dust resistant.
Check out Apple's comparison between the iPhone 7, iPhone 6s, and iPhone SE.