The Unified Interface client allows users to access Dynamics 365 CE using their browser, just like the web client, but it also provides a user-responsive design experience. Users will have the same experience of using Dynamics 365 CE across devices of different screen sizes.

The following screenshot is of Unified Interface. Here, you can see the difference between the web client and the Unified Client layout: 

The Unified Interface is based on Power Apps model-driven apps, where users can bond to specific Dynamics 365 apps based on their job profile. All the new apps of Dynamics 365 use the Unified Interface client.