What is the difference between Power BI Desktop, Service, workspaces and apps?

• Power BI Desktop — the free Windows authoring tool. This is where you connect to data, transform it in Power Query, build the model, write DAX, and design the report. All real development happens here.
• Power BI Service — the cloud platform at app.powerbi.com. You publish to it, and it handles scheduled refresh, sharing, dashboards, subscriptions, alerts, and governance. Reports can be edited here, but the modelling experience is limited.

Within the Service:

• Workspace — a container for datasets, reports, dashboards, and dataflows, with role-based access: Admin, Member, Contributor, and Viewer. This is the development and collaboration space, and it is where your team works.
• App — a curated, published package from a workspace, distributed to consumers. This is the consumption layer. Users get a clean, read-only experience with only the content you chose to include, and you can update the workspace without affecting them until you republish.

The distinction that matters: give consumers an app, not workspace access. Workspace access exposes work in progress and makes permissions harder to manage.

Note: A dashboard and a report are also different. A report is multi-page and interactive, built from one dataset; a dashboard is a single-page collection of pinned tiles that can come from several reports. Mixing up these terms is a common giveaway.