BI is a concept encompassing many areas of IT and like many other IT terms, it means different things to different people. One simple definition of BI is "Using of analytic and visualization tools to better understand and interpret data." To better understand BI, get the "Getting started with business intelligence in SharePoint Server 2010," from the link below and see the poster below:
Notice there're three areas of BI, i.e. at individual, community, and organizational levels. SharePoint 2010 addresses these areas as a whole with various vehicles including: Excel and PowerPivot Add-in, Excel Services, PerformancePoint Services, Visio Services, and Reporting Services and Report Builder as depicted below. And it's important to keep context in mind of a BI scenario that's being assessed such that the best vehicle, namely right tools and best-fit features, will become evident.
Download: Getting started with BI in SPS2010