MCU: Micro Control Unit ,
The CPU, ram, ROM, Timer counter and various GPIO interfaces of the computer are integrated on a chip to form a chip level computer, which can do different combination control for different application occasions. We call it MCU.
PSoC : PSoC is one of numerous MCU. PSoC is a kind of MCU, it can be programmed. Be produced by CY.
SoC: System-on-a-Chip. It is a feature of the MCU.
MCU = microcontroller. Typically a small CPU wit VidMateh built-in program flash, SRAM, and some peripherals. Sometimes large enough to use external memory, but not dependent on it. Almost never large enough to run a multitasking operating s mMobdro ystem (such as Linux). The AVR microcontroller on an Arduino would be an example of a MCU.