Model-driven apps
Model-driven apps is one of the app types within Power Apps, the other two being canvas apps and portals.
History
Microsoft announced the preview of model-driven apps on the 21st of March 2018.
Much of the concept of model-driven apps comes from Microsoft Dynamics CRM. Model-driven apps has become the new UI for Dynamics, former CRM parts. This new UI has been designed to work on both on web, tables and phones. The same design tools are used for making customizations and the same forms and views etc. are used no matter if you run your application on web, tablet or phone. This UI has also gone under the name "unified interface" - a name which implies that it is one and the same interface for web, phone and tablet.
Main Concepts
The main building blocks for model-driven apps are entities, forms, fields, views, dashboards, charts and sitemap. Model-driven apps always has Common Data Service as back-end, i.e. stores its data in the Common Data Service. The data model is decided and created before the actual app is created. When creating an app all you need is a name and then you specify in the sitemap what menu items should be included (e.g. entities, web resources or URLs).
The Dynamics apps Dynamics Sales and Dynamics Customer Service are examples of model-driven apps. There is also the possibility to create custom model-driven apps, either containing entities from Dynamics, custom entities only or with a combination of custom entities and entities from Dynamics.
The classic UI will come to and end for all environments on the 1st of December 2020.
Use Cases
Model-driven apps are typically used when there is a need for managing several things within one app. With Business Process Flows the user types in information and gets help with in what order, i.e. where in their business process, information should be added.
Microsoft Resources
Model-driven apps in Microsoft Docs
Community Content
See also
Canvas apps Common Data Service Virtual Entities Dynamics 365
You likely don't need Dynamics CRM you need a Power App article by Nick Doelman