Warehouse management is the management of stock that is placed in very specific storage areas such as shelves, bins, racks etc.
SAP WM (Warehouse management) has full functionality including the ability to suggest the placement of the stock received into specific bins in tne warehouse, taking into consideration things like the weight and volume of the stock that the bin can hold and special considerations like keeping apart certain types of materials.
in inventory management the stock is only managed down to a storage location level.
So if you have stock that is managed in bins, shelves and racks etc. and you want SAP to manage the placement and removal of that stock then you need Warehouse Management, if not then you simply use SAP inventory management.