The two primary technologies that comprise the OPC standards are OPC Classic and OPC UA (Unified Architecture).
OPC Classic is the original architecture, first released in 1996, based on Microsoft Windows technology, using COM/DCOM (Distributed Component Object Model) which was the Microsoft standard for software application communication.
OPC UA integrates all the functionality of the existing OPC Classic specifications into a single service-oriented architecture, while adding new features to OPC, such as platform independence, complex information modeling, security, and reliability.