An arranged overdraft can be useful for short term borrowing. It allows you to go overdrawn up to an agreed limit. The arranged overdraft fee is not applicable to the Student overdraft. An unarranged overdraft is when you spend more money than you have in your current account, without already having an arranged overdraft limit.
Both arranged and unarranged overdrafts can cost you money in fees, but an unarranged overdraft will cost you more and might result in payments such as household bills being rejected.
An overdraft is a form of borrowing and is repayable on demand. If you don't pay it back, it could have an impact on your credit score and could result in household bills being rejected.