At the beginning of a reporting period, or after a cycle count, the stock available in your inventory account is the Opening Stock. It is also called as Beginning Inventory.

So, there's an Opening Stock. Then, lots of transactions happens - Items are purchased and Sold. And finally Closing Stock is calculated.

Closing stock is the amount of inventory that a business still has on hand at the end of a reporting period. This includes raw materials, work-in-process, and finished goods inventory. The amount of closing stock can be ascertained with a physical count of the inventory. It can also be determined by using a perpetual inventory system and cycle counting to continually adjust inventory records to arrive at ending balances.

Closing Stock is an asset. In Inventory Account, it is under debit. In trading Account, it is under credit. Because, it is still not traded.