In our simple model of the widget fabricator, the "network" is just the wire leading from the switch to the panel light. In real life, you want to be able to monitor a lot of systems from a central location, so you need a communication network. This a special kind of network used to transport all the data collected from your sensors ultimately to some kind of operations center.
Early SCADA network protocols communicated over radio, modem or dedicated serial lines. Today the trend is to put SCADA data on Ethernet and IP over SONET. For security reasons, SCADA protocols are frequently restricted to closed LAN/WANs to avoid exposing sensitive data to the open Internet.
Real SCADA systems don't talk with just simple electrical signals, either. SCADA data is encoded in protocol format. Older SCADA systems depended on closed proprietary protocols, but today the trend is to open, standard protocols and protocol mediation.
Sensors and control relays are very simple electric devices that can't generate or interpret protocol communication on their own. Therefore the remote telemetry unit (RTU) is needed to provide an interface between the sensors and the SCADA network. The RTU encodes sensor inputs into protocol format and forwards them to the SCADA master.
This exchange can be asynchronous with events reported by the RTU as they occur. It can also be exchanged using a command and response protocol that is generally controlled by the SCADA master.
If the RTU is designed with outputs, it can also receive control commands in protocol format from the master and translate them to the appropriate physical or communication outputs.