Nowadays there is an increasing variety of computers of different sizes and  designed  for  different  purposes.  So  computers  can  be  divided  into  3  broad categories:  mainframe  computers  (mainframes),  minicomputers  (minis)  and microcomputers (micros).

Mainframes are large, powerful, expensive computers that are operated by a team of professionals and are designed to be used by many people at the same time.  They  can  handle  large  amounts  of  information  very  quickly.  The  most powerful  mainframes  are  called  supercomputers  and  can  be  found  in  large institutions like universities or government departments. Minicomputers are about the size of an office desk and you can usually see them  in  banks  and  offices.  Nowadays  they  are  becoming  less  popular  than microcomputers.

The most common type of computer is the microcomputer, sometimes called personal computer. The abbreviation PC was originally used for microcomputers produced by IBM Corporation. However some other companies such as Apple Computers Incorporated which produce microcomputers are not normally referred to as PCs. There is a wide variety of microcomputers but two common types are desktop computers and portables. Desktops are small enough to sit on an office desk and are relatively cheap. They are becoming cheaper and more powerful and are often used for running multimedia programs i.e. programs that contain some combination of text, sound, high quality graphics, animation, and video. 

There are two fundamentally different types of computers: analog and digital.  The former type computers solved problems by using continuously changing data  such as voltage. In current usage, the term "computer" usually refers to high-speed digital computers. These computers are playing an increasing role in all branches of the economy. A digital computer is a complex system of four functionally different elements:

1) the central processing unit (CPU),

2) input devices,

3) memory-storage devices called disk drives,

4) output devices.

These physical parts and all their physical components are called hardware. The real power of a computer depends on the speed with which it checks switches per second. The more switches a computer checks in each cycle, the more data it can recognize at one time and the faster it can operate, each switch being called a binary digit or bit. The power of computers also greatly depends on the characteristics of memory-storage devices. Most digital computers store data both internally, in what is called main memory, and externally, on auxiliary storage units.  As  a  computer  processes  data  and  instructions,  it  temporarily  stores information  internally  on  special  memory  microchips.  Auxiliary  storage  units supplement the main memory when programs are too large and they also offer a more reliable method for storing data. There exist different kinds of auxiliary storage devices, removable magnetic disks being the most widely used. They can store up to 100 megabytes of data on one disk, a byte being known as the basic unit of data storage.