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The microcomputer is in fact a digital computer as its name would imply.
The heart of microcomputer is microprocessor. The microcomputer, like computer has random access memory (RAM) , read only memory (ROM) , input—output and control units.
The transfer of data between microprocessor, memory and input-output units are via data bus. The transfer of address to memory and input—output unit are via address bus.
The microprocessor instructions are designed as simples as possible in order to have the minimum electronics circuits, in such a way that a microprocessor can be put on a dual in line chip with about 40 pins.
Here is one of most famous microprocessor 8080 chip with data, address and control pins.
A word of data can be 4,8,12 and 16 bits, but 8 bits lengths is more commons.
The memory capacity is normally up to 64 k byte. But for normal application up to 8 k byte is sufficient.
As far as instruction is concerened, the microprocessor has limited instruction, from very few for example 13 instructions to 100 instructions.
Because of capability, versatility, smallness and cheapness of microcomputer it is widly used in special purpose applications and industrial control systems.