What's the difference between an arranged and an unarranged overdraft?

An arranged overdraft is an amount agreed with the which you may borrow up to on your current account. It is recommended that this is not used as permanent borrowing facility. The bank may, at it's discretion agree to make payments on your behalf when you have insufficient funds in your account. This may result in you being overdrawn without an agreed arrangement. This is an unarranged overdraft.