If there is not enough of an available balance in your account to cover a withdrawal or payment presented against your account, an overdraft will occur and fees may be assessed. These are called NSF fees or overdraft fees. You may also hear the term "insufficient funds" used.
An NSF fee is charged when a checking account is overdrawn causing a negative balance at the end of the previous business day. NSF fees are always charged the day after the overdraft occurs. You can avoid NSF fees by making a deposit to bring your account to a positive balance by the end of the business day.