generally in SRM business partner means supplier.customer 
means enduser.
business partner means those who supply the raw material to 
the cusomer.i.e.buyer.

1 - if take referace of SRM then, In SRM all the stake 
holders are partners. eg vendor, employees etc. 

  2 - Business partner is as above, Partner function -  
make vendor as ref, then there will be invoicing party, 
suppying party, etc these are the partner fuction. for 
customer ship to party, ivoicing party etc