SAP BW & SAP ABAP dataflow??

Hi Friends,

Am new to SAP BODS....I have few questions regarding SAP BW source and ABAP dataflow..

1.what is SAP-BW ??(whether its a application or database)

2. How the datas we will be stored inside SAP BW...(Files or Database or objects etc.,)

3.what is ABAP??

4.what is ABAP dataflow??(why they are using separate  flow for extracting data from SAP BW)

5. Why they are using Data Transporter as a target in ABAP dataflow ??

Thanks,