Power BI is a free, self-service analytics tool available individually or integrated with Microsoft Excel, Office 365 and all major Databases and analytics tools.
Power BI is a cloud-focused data integration and visualization service that gives you a single view of your data. Dashboards, reports and datasets are the core functional objects in Power BI.
The main components of Power BI are Power BI Desktop and the Power BI Web Service. Power BI Desktop is a design tool for integrating datasets and visualizing them in reports, then the Power BI Web Service focuses on sharing the reports and dashboards with users so that they can view and interact with them.
Power BI also has a range of APIs for programmatic access to the Power BI Web Service.
Power BI Desktop is available as a free downloaded from Microsoft.
For documentation, please see white papers.