To know the differences between PLC and SCADA, first we can see from the word of acronym:
PLC is Programmable Logic Controllers, a PLC is just a controller to do some works. It may be a “brick” with lots of terminals on it for wires and control something more or less by itself.
SCADA is Supervisory Control And Data Acquisition. SCADA is more complex and typically contains number of systems such as communications, data logging, server access, etc. SCADA may be used mostly by utilities and include data exchange requirement and protocols.
The difference between PLC and SCADA is also in the industry and applications.
Traditional PLC are migrating toward to traditional DCS for instants Fisher Delta V. For application in small or medium plant many people use a PLC (Allen Bradley) and also use DCS (Delta V, PlantScappe). If you have a small plant, it is better to set PLC or DCS system which has a system starting at 5 I/O and going up to 500 I/O.
SCADA is just that supervisory and data acquisition. It usually is not in plant but it used to supervise multiple small sites. In industry SCDA often do monitoring systems with little RTU at each well site.