Power BI V2 went GA last week. This means it's no longer in "Preview" status; it's "Generally Available." Preview offerings are not intended to be used in Production environments, so now companies can begin formally converting over from Power BI for Office 365 (V1) to the new Power BI service (V2) when they're ready.
Different Ways Power BI Can be Utilized
If you are just beginning to research Power BI or haven't needed to pay extremely close attention to its evolution these past couple of years, you might find information online to be confusing or contradictory. The way I usually explain it is that there's four ways Power BI could be deployed depending on which components you choose to use.
With option 1, you're using just the Excel add-ins (Power Query, Power Pivot, Power View) and/or just the Power BI Desktop application. Those files are saved and shared on the file system. Although it's not an end-to-end solution, this mode certainly does offer a lot of functionality.
Option 2 is the on-premises solution utilizing SharePoint. It's been around quite a while now. There are some meaningful differences between using full-fledged SharePoint vs. the online services.
Option 3, Power BI for Office 365 (aka V1), is being retired in favor of Option 4, Power BI Dashboards (aka V2).
When discussing Power BI these days (mid-2015), people are usually talking about #4 which is the end-to-end solution. Although sometimes they could be talking about just option #1 without the web-based component. #2 is still a viable solution; it's just not the "hot" thing right now & doesn't get talked about much.
My purpose for writing this post is this... Please, please pay attention to the age of the article or blog you're reading and which product it relates to. If you're reading an older page about V1, chances are at least some of the information isn't accurate regarding V2. If you're reading about Power Pivot for SharePoint, very little will line up with the Power BI V2 Software-as-a-Service model. Even reading about Power View functionality within Excel versus within the Power BI Desktop differs significantly.