Hi Marcio:

Qty Ordered : The quantity that was entered in the Sales Order

Qty Delivered : The sum of the quantities of the item from all the Delivery Orders that were completed against that Sales Order (after converting to back to the UOM in the Order, in case the Order UOM is different than the Inventory UOM)

Qty Invoiced : The sum of the quantities of the item from all the Invoices that were raised against that Sales Order

For example, when you place an order for 10 units of Product A, the Qty Ordered will be 10. Let's say you generate a Delivery Order and deliver 6 units (with 4 units on backorder), the Qty Delivered will be 6 at this point in time. The Qty Invoiced at this point will be 0. If you create an Invoice at this point based on delivered quantities, the Qty Invoiced will be set to 6. 

Now, if you process a second Delivery Order for the backordered 4 units, the Qty Delivered will be increased to 10 (6 + 4) but the Qty Invoiced will still be 6 since the Invoice for these 4 units has not been created. After you create the Invoice for the delivered backordered items, the Qty Invoiced will be set to 10 (6 + 4).