Introduction To Power BI

At a very high level, Power BI is a stack of two "companion" applications - Power BI Desktop and Power BI Service.

Power BI Desktop (formerly Power BI Designer): A standalone self-service BI tool/application that is freely downloadable from the Power BI site (available in 32-bit & 64-bit mode). Power BI Desktop is a data modeling and content authoring tool that unifies the power of Power Query, Power Pivot, and Power View.

Below are some of the important tasks we can do using Power BI Desktop:

• Discover, connect, extract, transform data from public as well as corporate data sources (this was done using Power Query).
• Create data models (this was done using Power Pivot).
• Create interactive, drag-and-drop visualizations and explore/navigate geospatial data on a map in just a matter of a few clicks (this was done using Power View and Power Map).
• Publish/Deploy data models and visualizations to Power BI Service (more on this below) with a single click. Power BI Service is where the published content will be available for other users in the organization.

Power BI Service: Power BI Service (aka PowerBI.com) is a cloud-based Business Analytics Service (SaaS offering) that provides a simple, interactive, and user-friendly interface to create, view, share dashboards/visualizations, interact with your data and collaborate across your organization. Users interact with the service using a web browser or a mobile application.

                

Below are some of the important tasks we can perform with the Power BI Service:

• Connect to a variety of data sources (on premise or cloud).
• Build rich, interactive reports and dashboards. 
• Interact with data using Natural Language Query (also called as Q&A) – This feature allows users to ask questions in plain English and Power BI will respond to the question with an interactive chart or table. Power BI will interpret the semantics of the question and display data in the form of a visual. This visual can be tweaked to fit the need or can be added to an existing dashboard.
• Leverage Content Packs – For popular SaaS systems such as SalesForce, Zendesk, GitHub, Dynamics CRM etc., Microsoft Power BI team has built a set of data models, dashboards, and reports and published it to the Power BI Service so that users can consume the models/dashboards/reports. This pre-packaged collection of data models, dashboards, and reports is called Content Pack. Simply put, Content Packs provide out-of-the-box connectors to popular SaaS sources with pre-built data model, reports, and dashboards; these dashboards and reports can be customized (“personalized”) if required to fit the needs. 
• We can also create our own “Organizational Content Packs” and publish them to the Power BI Service. More on this here.
• Stay connected from anywhere – users don’t need to be on the corporate network, all they need is a browser (or Power BI app if using a mobile device) and an internet connection.
• Upload Excel files (either stored locally or on OneDrive), PBIX files (build using Power BI Desktop) and create visualizations based on the data in the files.
• Share content (dashboards etc.) with other users in the organization. Users just need a web browser to access and interact with the visualizations. Users with mobile devices can access the Power BI content using a browser on the mobile device or use Power BI app (native apps available for iOS, Windows, and Android devices).
• Setup data-driven alerts and create annotations on mobile devices and sher with other users.
• Schedule data refresh for online and on-premises data sources. By default, datasets from content packs are automatically updated once a day, but can also be refreshed manually or based on a schedule.

If you are still not clear on what Power BI Desktop is for and what Power BI Service is for, Power BI Desktop vs. Power BI Service might help.

Extensibility: Power BI is an extensible platform and provides the following ways to extend what is available out-of-the-box.

• REST API: Power BI’s REST API library makes it a fully extensible platform for its customer to customize pretty much every component of Power BI, including connecting to custom data sources, enabling real-time data streaming from data sources to Power BI, and integrating other line-of-business applications with Power BI. 
• Open Source Visualizations: Microsoft has released an open source visualization project on GitHub to enable Developers to start building custom visualizations. This is an excellent move towards extensibility as customers will be able to extend out-of-the-box visuals and are not limited by what’s available within Power BI.
• Custom Visuals Gallery: Microsoft lets Power BI community create and deploy custom visualizations to https://app.powerbi.com/visuals/, where other community members can download the visuals from and use it, free of charge.

Hope this is helpful.
Note: Images referenced in this blog post have been taken from Jen Underwood's Power BI Technical Reference Deck
Related links:

Power BI Basic Concepts 

Power BI Pricing 

Power BI Licensing Revisited