A computer system is usually composed of a Central Processing Unit (CPU), input devices, storage devices and output devices. The CPU includes an arithmetic-logic unit (ALU), registers, control section and logical bus. The arithmetic-logical unit performs the arithmetic and logical operations. Logs store data and results of operations. The control unit regulates and controls various operations. The internal bus connects the CPU units with each other and with the external system components. On most computers, the main input device is the keyboard. Storage devices are hard drives, flexible (floppy disks) and compact (CD). Output devices that allow data to be viewed are monitors and printers.
All modern digital computers are conceptually similar regardless of their size. However, they can be divided into several categories according to their price and performance: the computer or personal computer is a machine of relatively low cost and usually of adequate size for a desk (some of them, called portals, or laptops, are enough small enough to fit in a briefcase); the workstation, a microcomputer with improved graphics and communications capabilities that make it especially useful for office work; the mini-computer or mini-computer, a larger computer that is usually too expensive for personal use and is suitable for companies, universities or laboratories; and the mainframe, a large, high-priced machine capable of serving the needs of large companies, government departments, scientific research institutions and the like (the largest and fastest machines within that category are called supercomputers).
In reality, a digital computer is not a single machine, in the sense that most people consider computers.
It is a system composed of five differentiated elements: a CPU (central processing unit); input devices; memory storage devices; output devices and a communications network, called a bus, that links all the elements of the system and connects it to the outside world.