By saying, perpetual or periodic inventory system; what do we mean?

 Periodic and perpetual   inventory systems are different methods used to track the quantity of goods on hand. The more sophisticated of the two is the perpetual system, but it requires much more record keeping to maintain.

The periodic system relies upon an occasional physical count of the inventory to determine the ending inventory balance and the cost of goods sold, while the perpetual system keeps continual track of inventory balances.

They differ in so many areas , I will touch upon one area that is Accounts : Under the perpetual system, there are continual updates to either the general ledger or inventory journal as inventory-related transactions occur. Conversely, under a periodic inventory system, there is no cost of goods sold account entry at all in an accounting period until such time as there is a physical count, which is then used to derive the cost of goods sold.