OPC UA replaced the Windows-bound OPC Classic with a platform-independent framework that does two things. It defines how clients and servers talk, with built-in authentication, encryption and session management, and it defines an information model so that a machine can describe its own structure: a motor has a speed, a temperature, units and a history, rather than just a numbered register. Companion specifications standardize those models for sectors such as robotics, machine tools and packaging.
In industrial IoT it is the bridge between PLCs and the IT world, feeding SCADA, MES and cloud analytics. The PubSub extension lets OPC UA data flow over MQTT or UDP for many-to-many scenarios. The costs are complexity and footprint: a full server is heavier than Modbus, though lightweight profiles exist for small controllers. The industrial IoT guide shows where it fits.
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