PLC

PLC is a Robust Industrial computer. Easy to operate, simply runs codes given to it and can handle lots of inputs & outputs.

SCADA is a Control, surveillance & automation execution design. It’s a plan of how to set up Industrial automation, a surveillance system to check remote units in a plant & take control if something goes wrong by using surveillance computers which are connected to devices in plant’s units.

SCADA has 5 components which speak of elements needed to achieve such a system. They include sensors, alarms(to gain the working state of a unit in the plant),WLAN & LAN, OPC servers(for the purpose of communication between sensors and the surveillance team in control room), PLC(a computer to process signals from various sensors on field and present a simplified information about the unit's status & such), RTUs which are wireless control units, HMI (human machine interfaces where we can check n interact with an interface and manipulate the working condition of a unit when need be)

So, we are talking about a computer and a control system architecture. Which are way different.