Clarification: Extraction from R3 to BW

Hello,

Can you give all the steps and transactional codes (involved to see the table on R3 and to retrieve data) if I have to retrieve data from the standard general ledger extractor to BW environment?

The extraction is supposed to be delta with Fiscal Year/Period as selection criteria.

By the way, what actually an extractor? A program or just a table of fields?

I have already seen different SAP links and I have the general idea but can you directly answer my question if you were in front of a computer and needs to implement what I described?

Thanks.