Work Orders are used to consume raw materials and make finished goods.  Sales Orders and Invoices are used to ship items to customers.  
 
If you are selling a component by itself, you would use only the Sales Order and process the invoice.  Posting the invoice will take the item out of stock and reduce the on-hand quantity of that item, there is nothing else you need to do.
 
If you are selling an assembly that has a BOM, posting an invoice will ship the assembly to the customer and reduce the on-hand quantity of the assembly but you need to first use a work order to consume the components and then put the assembly into stock.  The invoice will not do anything with the components.