Work orders and invoices are both part of the paperwork businesses use when assigning and paying for projects. A work order lists and assigns the necessary tasks. The invoice is the bill for the work.
A work order is a formal written request. It spells out exactly what needs to be done, who is supposed to do it and who gets the bill. The Oracle Help Center says a work order can be external -- something one company sends to a contractor -- or used in-house. An accounting department, for example, might open a work order with the IT department to get some computer upgrades done. Companies usually have a system for filing and tracking work orders.
There's no need for an invoice when work is done in-house. The invoice comes into play when an outside contractor does the work. The Accounting Coach website says the contractor sends an invoice that breaks the work down into individual tasks, prices per task and the total charge. It also states the terms, such as due in 30 days.