sap bw is an seperate system, you will transfer data/structure from R/3 to bw for project requirements. bw is a seperate system, there you will do modeling, extraction, reporting seperately like in R/3.
Open the link and follow left side frame sets, all those activities and content we will do in sap bw
http://help.sap.com/saphelp_nw70/helpdata/en/b2/e50138fede083de10000009b38f8cf/frameset.htm
sap is designed for data warehousing, It is like all systems designed for doing that.