You seem to be confused.....
A PLC is a complete system that contains a processor, I/O ports power system and possibly other hardware that may be required for its proper operation.
A PIC (Peripheral Interface Controller) on the other hand is just a micro-controller, depending on the part number it can be 8 bit, 16 bit or 32 bit or 5 V / 3.3 V etc.
A analogy for the above description would be a car and an engine, a car like the PLC contains apart from the engine other components that are essential to make it work, for example: gearbox, a body, a battery etc.
The engine can be compared to the PIC. An engine cannot be used by itself, it needs other hardware to make it work properly, in the same way the PIC needs a power supply, extra hardware for interfacing to a PC or some other devices like ADCs or DACs etc.
I hope this was clear enough.
Visit
Microchip Technology Inc. - a Leading Provider of Microcontroller and Analog Semiconductors for more information on PICs, you could also visit
CCS, Inc. - Home
thanks
arun