Where the order-management-system cares about correctness and record, the EMS cares about how to trade. It hosts algorithm containers, broker connections, live execution-quality measurement and the controls to intervene mid-order.
Many vendors sell combined OEMS platforms, but the functional split survives: one component is the book of record, the other is the trading cockpit.
Example: a 300,000-share parent arrives from the OMS. The trader starts a vwap-algo at a 12% participation-rate, watches realised slippage run 9 basis points behind arrival, switches to a liquidity-seeking-algo for the final 80,000, and finishes 4 basis points inside the arrival-price.
Related: order-management-system, algorithmic-order, smart-order-router, execution-quality