In the Western world, Saturdays and Sundays are not counted as business/working days. In some Muslim countries and in Israel, this applies to Friday and Saturday.
Business days are common measurements in companies with time-sensitive services, such as shipping or payment processing. As an example, when sending a package with a shipping company such as FedEx, a shipment dispatched on a Thursday that is to be delivered in "two business days" will arrive on the following Monday.
In the United States of America, a work day is generally referred to as a weekday, not including Saturday and Sunday.