1) under double entry system every transaction has two aspects Debit & Credit & at the time of recording it is once recored on debit side and then on credit side also every time the amount is same on both sides,
2) in journal proper we record the entries which are not related to cash, sale ,purchase sales return, purchase return b/p, b/r . the entries recorded are which are not of this kind
eg: discount
4)it is the discount allowed for timely payment of due amount it is an expense for party allowing the discount & income for the recieveing one.
5) debtor is the one who takes place due to credit sale . ddebtor is a person who owes money to firm.
and sry for not giving answer for concpet coz i m nt gettin it .. for which concept u r asking for ?