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

Under Periodic Inventory system, no detail record of movement (in & out & balance) is maintained during the assigned period. Actual physical inventory count is done at the end of each period. Periodic systems is mostly used when inventory consist of different number of products of low value. 

Where as in perpetual inventory system detailed record of in and out (based on each transaction) is maintained within accounting or WMS system. It is used when inventory items are of high value.