PLC: programmable Logic controller. the brain that conrol the function of machine parts, it was made to replace relays, timers and counter etc.
Scada: Super-visory Control And Data Acquisition. it is the software design that allow you to read values from PLCs through a PC monitor, it also allow you to write register values and read register value then programmatically visualize them graphically. etc. many engineers consider machines with Scada interface: CNC machines.
HMI: Human Machine Interface. similar to Scada, it is an LCD display usually rectangular in shape and it has function buttons on it that are connected to internal PLC bit to triger outputs. difference between HMI and Scada is that Scada is PC based monitoring system whereas HMI is eletronic and does not take alot of time to restart like PCs
I dont know much about DCS however I hear they are similar to plcs.. above is a brief explanation only and does not include all of the functions involved... as every one else suggested; do your research and you will know better.
the best way to learn PLC is to buy a PLC , a software, programming cable and alot of panadole tablets..(ADVIL isnt bad as well) and start the fun.
hope this help
