Power BI is not a single tool but consists of multiple tools and additionally building blocks.
Power BI Desktop is a local application, where we can create our datasets, visualizations and reports. We then publish the information to Power BI Service, which is a cloud solution. In there we can collaborate with our colleagues or add dashboards to our existing data. So, this way we have the data in the cloud. Now, if we use the third tool, i.e. Power BI Mobile, the mobile app will allow us to access the data that we loaded to Power BI service, from our mobile devices.
So, Power BI Desktop, Power BI Service and Power BI Mobile the three core tools. However, there is Power BI for Developers as well. This is made to customize the customer experience, which means that you can create your own customized visuals and you can also embed the Power BI results into your own existing apps.