An early calculating device called abacus was developed more than 2000 years ago. It is still in use in some countries. Several matherhaticians and scientists have worked during the last three centuries to develop computer.

The early electronic computers, called the first generation computers were based on vacuum tubes. The second generation was based on transistors, and the third generation computers were based on ICs. The fourth generation computers are based on LSI and VLSI ICs.

One of the most commonly used classifications of computers is based on its size. Here, the computers are divided into three categories-Large (mainframe), mini and micro. The PCs that you use fall in the micro category. Large companies, banks, airlines, etc. normally use large (main frame) computers. The mini computers fall between the mainframes and micro computers.

Computers are also be classified as analog and digital. PC is a digital computer. The computers used in offices and homes are generally digital. Digital computer operates directly on numbers that are expressed as digits. Digital computers are more accurate than analog computers. They can be used for data processing as well as for scientific, engineering and industrial applications. On the other hand, analog computers that are normally used in the process industry or in instruments can directly measure physical parameters, such as voltage, current pressure, temperature, length, weight, flow etc. However, now-a-days, digital computers are also being used for industrial applications.