Microcomputer
A microcomputer is the original terminology used to describe a personal computer (PC). This would encompass all PCs - from the smallest to the biggest, most powerful to the least powerful. Portable or desktop. Typically, a personal computer is designed to have only one user at a time logged into it and using it.
In contrast;
A midrange computer is a small, but pretty powerful multi user machine that is often refered to as a server (like an application server or a web server). To a limited extent, these computers can be broken into several smaller virtual machines.
A mainframe is a monster of a computer and can accomidate thousands of users at a time. Enormous processing power and extensive virtualization (many smaller virtual machines are created from one physical machine).
A super computer is just that - total pie in the sky computing. Mainly used in specialized research venues.