customer debit and credit note

Hi Friends,

clarify my doubts...

1) what is customer debit note and credit note.

2)what is the doc type for debit note and credit note.

3) what are the posting keys for it

4) what are the transactions for both note.

I want to post some transactions for both customer debit and credit note from FI point with VAT..

please solve my issues ...

Regards

Nandu Savi