Each equipment has a communication protocol. This protocol defines the bytes that will be sent and received to the SCADA application. The driver is a piece of software that converts these bytes to the information.
Generally these equipment are PLCs (Programmable Logic Controller). The communication protocol has several layers. Application, presentation, pivot session, transport, network, data link and physical.
There is three most common types used in industry: serial, ethernet and wireless.
The serial communication can be RS-232, RS-485 or USB.
Ethernet communication is most commonly used with a TCP protocol.
In the application level communication protocol there is some standards like MODBUS or Ethernet/IP.