SRM SUS EBP

Hi i'm totally new to SRM. i'am so confused with it, that i can't proceed with the development requirement (work flow).

I want to know :

     what is EBP, SUS and are these systems not the same (physically different) ?

      what documents are created in SRM and what documents are created in ECC, how are they connected ?

      what is the difference between EBP,SUS,SRM ?

       what is follow on document (googled already, but no proper answer). ?

       Where is the PO created, SRM or EBP or SUS or ECC ? and if so why can't we save the PO in SRM itself ?

Please don't ask to go through any previous threads, already searched for the last 1 month.

Thanks in Advance, for explaning me.