Elayne Jones gives us an introduction to Power Apps:
The starting point for working with Power Apps is an environment. Environments house your business’ apps, data, chatbots, and flows. By building apps in a single environment, users can isolate content aimed at a specific use case or target their content towards a specific team or department. A common practice is to build separate environments for Development, Test, and Production stages. Power Apps Environments can even connect to GitHub, streamlining source control within an organization.
An Azure Active Directory tenant is required to create an environment, and only users within that tenant can access the content within the environment. After the environment is created, users deploy data sources to that environment. Thereafter, the content created can only connect to the data sources within the same environment. You can create a database in each environment, but there can only be one database in each environment.
Read on for a lot more.