Power BI Desktop
The differences between the two are purely cosmetic.
Anything you build in an Excel Data Model can be converted to a Power BI Desktop Data Model with just a few clicks – because they use the same engines and languages! (Some of the visuals might not translate over, but those are quick to recreate. It’s the brains of your models that are important, and those transfer effortlessly.)
CREATE with Power BI Desktop
Power BI Desktop is where you CREATE models and dashboards.
SHARE with PowerBI.com
PowerBI.com is where you PUBLISH and SHARE share those models and dashboards. Then, others can view and interact with those dashboards in their browsers and, even better, on their mobile devices.
The answer is the SAME for Excel Reports vs. Power BI Reports– you BUILD either in their respective desktop app (Excel or Power BI Desktop), and then you can PUBLISH them to PowerBI.com. Excel reports get published to a section called Workbooks, and Power BI Desktop reports get published to a section called Reports.
But with Excel reports, you also have the option of “on premises” SharePoint as your publishing platform, so if you’re not ready for the cloud, this might be the right path for you.
What is PowerPivot?
Explains why Excel PowerPivot is such a life-changer over traditional Excel alone. (Yes, it’s written from the Excel PowerPivot perspective, but it still explains the original and most central of the Data Model Engines – the DAX formula and modeling engine – in a manner that’s relevant to Power BI Desktop.)
Transitioning from Excel PowerPivot to Power BI Desktop
Drills down on the similarities between the two environments. For more on the strategic reasons why Microsoft has two environments, see What is Microsoft Up To?