journal voucher is the voucher in which all the adjustment related entries and non cash non bank transactions are entered in journal eg-dep, some of them book the bills in journal and

while they make a payment they record in payment eg-contractor bill

contra appears two times in two sides of a account an account will be treated as contra when

1, cash deposited in bank

2, cash with drawn from bank for office use

3, cheques deposited in bank

4, cheques withdrawn for office use

5, transfers from one account to another account