COD orders are orders that were completed using the payment method 'COD/Pay Later' and will be paid later by the customer.
On your Sales Reports, orders that were originally COD but have been paid will show under the payment method that was used on the date payment was received.
If you want to see orders that were placed using COD and are still unpaid, check the box next to "COD/Pay Later" then Show Report and all orders still unpaid and marked COD will appear under the 'COD/Pay Later' section of the report.
Orders that have been paid for initially with "COD/Pay Later" will have the text COD/Pay Later" next to their order number regardless if they have been paid for with another payment method.
Please note: The Product Category Report will only show the order that has been paid, so COD orders will show on the report the date they are paid.