Model-driven apps
Model-driven apps is one of the app types within Power Apps, the other two being canvas apps and portals.
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, with custom entities only or with a combination of custom entities and entities from Dynamics.
You might also be interested in
Canvas apps Common Data Service Virtual Entities Dynamics 365
History
Much of the concept of model-driven apps comes from Microsoft Dynamics CRM.