A desktop computer is a personal computer (PC) designed for use in a fixed location, as opposed to a portable computer or laptop. The original desktop computers lay flat on the desk, usually with the monitor sitting on top of them. More modern systems use a tower case, which often sits under the desk rather than on top of it. The first desktop computer was the IBM PC, launched in 1981. Other desktops are often described as “IBM-compatible”.
Desktop computers generally have an external keyboard, monitor and mouse. The other hardware components are located inside the tower. Desktops are normally cheaper than a laptop with a similar configuration as the smaller components used in a laptop are more expensive. They usually have extra space within the tower for expansion and are relatively easy to upgrade.
An all-in-one computer is a type of desktop which has the hardware components located within the monitor case.This reduces the amount of space required, but they can be difficult to repair or upgrade.They are often equipped with touch screens.
For further information see the Wikipedia article on desktop computers.