Minicomputer

Minicomputer is a mostly outmoded term for a class of multi-user computers which generate the middle range of the computing spectrum, in between the largest multi-user systems (traditionally, mainframe computers) and the smallest single-user systems (microcomputers or personal computers). Formerly this class created a distinct group with its own hardware and operating systems notably having smaller address space (notice the cited numbers of bits in a data word, ranging from 8 to 24 bits commonly around 16-bits). While the distinction between mainframe computers and smaller computers residue fairly clear, contemporary middle-range computers are not well differentiated from personal computers, being typically just a more powerful but still compatible version of a personal computer. More modern conditions for such machines consist of midrange systems (common in IBM parlance), workstations (common in Sun Microsystems and general UNIX/Linux parlance), and servers.

Microcomputer

A microcomputer is mostly often taken to mean a computer with a microprocessor (µP) as its CPU. Another common characteristic of these computers is that they take up physically small amounts of space. Desktop computers, video game consoles, laptop computers, tablet PCs, and a lot of handheld strategy may all be considered examples of microcomputers. Most microcomputers serve only a particular user at a time, but some, in the form of PCs and workstations running e.g. a UNIX(-like) operating system, may cater to a number of users concurrently. The µP does the greater part of the job of calculating on and manipulating data that all computers do.

Mainframes

Mainframes (often colloquially referred to as big iron) are enormous and expensive computers used mostly by government institutions and large companies for mission critical applications, usually bulk data processing such as censuses, industry/consumer statistics, ERP, and financial transaction processing.

The term originated during the early 1970s with the introduction of lesser, fewer complex computers such as the DEC PDP-8 and PDP-11 series, which became known as minicomputers or in a minute minis. The industry/users then coined the term "mainframe" to describe bigger, earlier types (previously known simply as "computers").