PLC (Programmable Logic Controller)
Device used for industrial control. Typically these devices have 24V DC inputs and outputs. (Some have provide analog, encoder and similar interfaces) Beside the inputs and outputs they have some kind of CPU and program memory for holding the application.
Some devices offer various communication means(for example for linking to upstream control and HMI systems or for controlling remotely located inputs and outputs:
Very common PLCs are the SIEMENS S5 (now the S7) Series