Overdraft is a short term loan, allowed on the card account of the costumer. Sometimes, overdraft is called thirteenth salary and is launched corresponding to the salary (as a rule, amount of overdraft is 90% of the salary, though in some banks it could be more than salary). Usually, overdraft term is 1 year, only accrued interest rate is cut off from monthly transferred salary and principle sum is covered at the end of the term.
Overdraft allows the customer to use the sum, more than the customer deposits/owns. Maximum sum is verified individually, based on the credit history and income of the customer. Overdraft is almost similar to credit card though, the difference is that it is attached to the salary and privileged period doesn't affect on consumed funds.