SAP business warehouse offering or BW for short... well as with all things SAP the current favourite term is Business intelligence or BI for short which was a more generic term for the BI platform/tools that allowed companies to achieve what they wanted from BW. But anyway I digress; BW is basically a data warehouse which essentially contains a copy of your live data updated from your SAP R/3 or any other source at periodic times. Out of the box SAP BW provides many preconfigured data extractors and tools to allow you to get data from your SAP system into your Business warehouse info cubes! SAP BW can not only extract data from your R/3 SAP system but can also extract data from any non-SAP system using a number of methods ranging from file uploads to Business application interfaces ( BAPI's ).
Once the data has been imported into the BI system you have a number of tools and techniques that allow this data to be reported on in a very flexible way and by using multiple formats such as excel or a web based user interfaces. This allows business users to create a report that delivers them the information they need presented how they want it!