The CPU or the internal processor is where the instructions contained in a PLC program are executed.
Thus the size of the CPU is what defines how long it will take to execute (run) a specific program in the PLC.
Thus, a small PLC type has a small CPU, and a large PLC type both have a big and expensive CPU.
RAM (memory) holds a hardware of the PLC program and the operating system referred to as a firmware.
Electrical parts inside any PLC can be interfaced with the PLC program by the help of firmware to make all of them work. In addition, the firmware controls also the writing of values from the input modules and the giving of new values to the output modules.
The firmware lacks user interface in the sense of icons as people are familiar with on a PC or a smartphone.
Almost all PLCs consist of memory resources which are capable of being augmented if the size of the PLC program enlarges. Other types of PLC are programmable digital input/output; these are programmable with a removable storage card (SD card) where is stored the program and functions as a USB key.
ALU stands for arithmetic logic unit and this is a part of the computer where the calculations occurs.
Digital clock CLK is used where the PLC program requires to know about the time or where there is programming required.