OPC is a protocol developed for create a common way to transfer data and command from different PLCs.
More info are availables from the link below
OPC foundation
More info in Italian language are here.
The basic principle of OPC specification is a client-server model in which a any process (client) based on OPC can access any data source (server) with OPC interfaces.
In particular:
• OPC server enables hardware vendors to offer their customers services that allow any client to access their equipment
• OPC client application controls the devices and manage the relevant data using the standard methods of access to an object OPC