SRM, R/3 and BW

Hi Experts,

i am quite confused about the way how SRM is ntegrated with BW. can some one answer my basic questions

1. whats the main use of SRM

2. whats the difference between b/w R/3 and SRM. I mean, when we have PO/PR's etc why again SRM

3. where do we create PR's and PO's.

4. where does we maintain Vendor data. ( do we have in r/3 and then we replicate to SRM, if so where in SRM,(T-code)

5. what is a shoping cart and how is it related to R/3 and BW.

I assure you, you will get good points.

Kedar