A brief description of the different divisions is available from Wikipedia:
BT Group is organised into the following divisions:[53]
Customer facing: BT Global Services – provides IT and telecoms services to multinationals
BT Business – provides telecoms services to UK SMEs
BT Consumer – provides retail telecoms services to consumers including TV and BT Sport
BT Wholesale – operates BT's networks
Openreach – fenced-off wholesale division, responsible for the "last mile" of BT's access network in GB and tasked with ensuring that rival operators have equality of access to BT's local network
Internal service unit: BT Technology, Service & Operations – responsible for the innovation, design, test, build and running of BT’s global networks and systems.
or on the BT Group Website
http://www.btplc.com/Thegroup/Ourcompany/Groupbusinesses/index.htm