Inventory can be of several different types depending on how much value a process has added to it.
Raw Material is inventory that has had no value added to it since its arrival.
WIP is the inventory accumulated before, within or following the operations of a process.
Finished Goods is the completed work awaiting payment.
WIP Inventory includes buffer stock and safety stock, though buffer stock may also be called Finished Goods if located just before shipping. Standard WIP is the desired amount of WIP for that point in the "journey." It consists of parts being processed in machines and buffer stock held between operations.
Inventory is considered to be one of the worst of the 7 Wastes because of the way inventory costs compound; stock held in inventory not only ties up capital but requires additional expense of movement and storage.