Inventory parts are items that you buy and then stock for a while before selling. QuickBooks tracks the quantity on hand, the current inventory value, and the average cost of your inventory items. It also tracks some accounting information like "cost of goods sold" for inventory items that you have sold.
Non-inventory parts are items that you purchase (usually on behalf of a specific customer) and then immediately sell or install, or items that you sell without ever buying. For non-inventory parts, QuickBooks will only track how much you have spent or taken in. Some people use non-inventory parts even for items that they stock because they prefer a simpler approach—even though it gives them less information.