PLC "Programmable Logic Control" looks like a computer with a memory called "EPROM".The sensors like photocells, proximity switches and limit switches give inputs to that computer.
Upon the program stored on it,It gives outputs to motors, solenoid valves to actuate pneumatic cylinders.
This is simply PLC.
Mechanical Engineers who are going to work on production lines which is fully automated will deal with PLC and should have a solid background about it in order to help in troubleshooting such common problems between mechanical and electrical parts.:)