Both of these are cost objects which collect production costs for manufactured product. Product cost collector is a single order created for a material. All the costs during the month for that material is debited to single product cost collector. No costing by lot size is required in case of product cost collector.
The latter is where there are many production orders for a single material during the month. Costs are collected on each of this production order. Costing by lot size is the main requirement in case of production orders.