According to Microsoft: "Power Apps is a suite of apps, services, connectors and data platform that provides a rapid application development environment to build custom apps for your business needs. Using Power Apps, you can quickly build custom business apps that connect to your business data stored either in the underlying data platform (Microsoft Dataverse) or in various online and on-premises data sources (SharePoint, Microsoft 365, Dynamics 365, SQL Server, and so on)."
PowerApps is a low-code development system.
Microsoft Dataverse provides you with free data storage for your PowerApps solutions. Dataverse is a secure, and scalable low-code data platform. Dataverse uses built-in standard tables, extended attributes and an open ecosystem enabled by Microsoft's Common Data Model. The Microsoft Dataverse service is used for most Microsoft applications, including the entire Dynamics 365 suite.
There are two types of apps that can be created in PowerApps: Canvas and Model-Driven Apps.
A Canvas App, as the name suggests, allows the designer to drag user interfaces elements onto a blank canvas. Canvas apps allow designers to create a highly customised and unique user interface. Once all UI elements are in place, basic Excel-like formulas are used to connect UI elements to an underlying data source. More than 200 such data-source types are supported.
Model-driven Apps are more closely tied to data models in the underlying Dataverse. Model-driven apps take a data-first approach. User interface elements are pre-determined by the underlying data type and customisation is limited. However, in contrast to canvas apps the data-first, model-driven approach allows the creation of more complex solutions that support more sophisticated business logic. Model-driven apps are Responsive by default.