Difference between Zilog Z80 and Z84?
The original Z80 was produced in the older NMOS technology, the Z84C00 is its CMOS version. The CMOS version is able to run at higher clock frequencies.
The Z80C00 is the CPU, the Z84C20 is a PIO (Parallel I/O) device, and the Z80C30 a CTC (Counter/Timer Circuit). Nowadays all microcontrollers include functions like PIO and CTC on chip. This is the main difference between a microprocessor and a microcontroller.