It would be too easy to say the Samsung Galaxy S9 is just a carbon copy of the Galaxy S8... but it's very, very close.
There are a few slight changes that make a big difference though, starting with the materials used to make this phone. The fact it's using Gorilla Glass 5 is a big step forward, as it means the phone is even better able than its predecessor to withstand knocks and drops, removing the need for a case if you're not into encapsulating your new expensive phone.
The aluminum used in the construction of the device is also more robust, so less twisting is possible should you accidentally feel the need to warp your phone.