Loans provided by the banks have fixed loan amount and tenure. The interest rate on a loan could be fixed or floating type. When you take a loan, you have pay the entire loan amount with interest whether you have spent the entire amount or not.
In an overdraft facility, the bank allows you to withdraw money or write cheques against a certain amount from your account. The interest rate is calculated only for the amount withdrawn and not for the amount kept unused. It is flexible than a traditional loan.