A Warehouse Execution System sits between the WMS and the equipment, orchestrating which task goes to which resource in real time — balancing load across shuttles, conveyors and pick stations as conditions change minute to minute, something a WMS's order-level logic isn't built to do.
A Warehouse Control System is the layer below that, talking directly to PLCs and drives — the actual motor commands that move a conveyor or position a crane. In smaller deployments, WES and WCS functions sometimes collapse into one system; in large multi-vendor facilities they're usually kept distinct.