A:Answer I came from the Galaxy S6, and didn't follow all the interim changes to the Galaxy S line very closely. To my understanding, these are the significant changes from the S9 to the S10(standard models):
Display
S10: 6.1" HDR10+ Certified Infinity-O 19:9 3040x1440 AMOLED
S9: 5.8" Infinity Edge 19x8.5 2960x1440 AMOLED
Authentication
S10: Facial Recognition(Camera only), In-Display Ultrasonic Fingerprint Scanner, pattern, pin, passcode
S9: Iris Scanner(IR sensor and Camera), rear Fingerprint Scanner, pattern, pin, passcode
Cameras
Rear
S10: (3) Wide 12MP, Ultra Wide 16MP, Telephoto 12MP
*The Wide and Ultra Wide lenses feature OIS and Dual Pixel AF, the Wide features Dual Aperture(F1.5-F2.4)
S9: (1) Wide 12MP with OIS, Dual Pixel, Dual Aperture
Front
S10: Wide AF 10MP F1.9
S9: Wide AF 8MP F1.7
Standard Internal Memory
S10: 128GB
S9: 64GB
Processor/RAM(US Models)
S10: 8-Core Snapdragon 855/8GB RAM
S9: 8-Core Snapdragon 845/4GB RAM
Durability
Both: IP68 Certified(dust and water resistant up to 1.5m of freshwater/30 mins)
S10: Gorilla Glass 6 front, Gorilla Glass 5 rear
S9: Gorilla Glass 5 front and rear
Battery
S10: 3400 mAh w/Wireless Powershare, Wireless Rapid Charging
S9: 3000 mAh w/Wireless Rapid Charging
Shipped Android Version
S10: 9.0 Pie
S9: 8.0 Oreo
I know it's just a list of specs, but review videos available on YouTube can tell you more. Hope this helps!