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Introduction to Computer
 A Computer is an electronic device that accepts data from the user,  processes the data by performing calculations and operations on it and generates the desired output as a result. The term computer is derived from the Latin word ‘Caomputerae’ which means ‘to compute’.

Computer is the combination of Hardware and Software which converts data into information. Computer operates on set of instructions only, they cannot think as human being.
Input : Information or data that is entered into a computer is called input. It sends data and instructions to the Central Processing Unit (CPU).

Processing : It is the sequence of actions taken on data to convert it into information which is meaningful to the user. It can be calculations, comparisons or decisions taken by the computer.

Output : It makes processed data available to the user.

Storage : It stores data and programs permanently.

Hardware : Hardware is the collection of physical elements that constitute a computer system. It is a comprehensive term for all the physical parts of a computer. E.g., display screens disks, mouse, keyboards, mouse, chips, printers scanner, etc..

Software It is a set of programs and procedures. Software tells the hardware what to do and how to accomplish a task. E.g., Web browsers, word processors, etc..

Data : Unprocessed raw facts and figures, like numbers, text on pieces of paper, are known as data.

Information : When data is processed, organized, structured or presented in a given context so as to be useful, then it is called information.

Instruction : It is a command given to a computer in the computer language by the user.

Program : It is a set of instructions given to a computer in order to perform some task.
                Classification of Computer

Based on Size
1.Microcomputer : Microcomputers are the least powerful, yet the most widely used and fastest growing type of computers and are also called portable computers. Microcomputer consists of three basic categories of physical equipment that is system unit, input/output and memory.

Personal or Desktop Computer (PC) These are small, relatively inexpensive computers. These are based on the micro-processor technology (Integrated Circuit).

Handheld Computers or Palmtops These are the smallest and are designed to fit into the palm. So, these are also known as Palmtop/PDA. They are practical for certain functions such as phone books and calendars. They use the pen for input instead of keyboard.

Smart Phones : Smart phones are cellular phones that function both as a phone and as a small PC. They may use a pen or may have a small keyboard They can be connected to the internet wirelessly. Apple, Blackberry, Nokia are some manufactures of smart phones.

2.Mainframe Computer : Mainframe computers are those having large internal memory storage and comprehensive range of software. Mainframe computer serves as a backbone for the entire business world. It is considered as the heart of a network of people to work at the same time. Mainframe computers are IBM-370, IBM-S/390, UNIVAC-1110

3.Minicomputer :  Minicomputer are smaller in size, faster, cost lower than mainframe computers. Initially, the minicomputer was designed to carry out some specific tasks, like engineering and Computer Aided Design (CAD) calculations. But now, they are being used as central computer which is called as sever. Mini computers are IBM-17, DECPDP-11, HP-9000,etc.

4.Supercomputer : Supercomputers are the fastest and the most expensive machines. They have high processing speed compared to other computers. The speed of supercomputers are measured in FLOPS (Floating Point Per Second). Super computers are used for highly calculation intensive tasks, such as weather forecasting, nuclear research, military agencies and scientific research laboratories. Supercomputers are most poweful, large in size and memory, compared to all other computers.
Based on Working of System

Analog Computer : Analog computer are the job-oriented computers. They carry out arithmetic and logical operations by manipulating and processing of data. E.g., speedometers seismograph, etc.

Digital Computer  digital computers work by calculating the binary digits. A digital computer, not only performs mathematical problems, but also combines the bytes to produce desired graphics, sound. e.g., desktop (PC).

Hybrid Computer :  Hybrid computers are the combination of analog and digital computers. Machines used in hospitals like ECG and DIALYSIS are the commonly used hybrid computers.
Based on Purpose:

General Purpose Computer  :  General purpose computers are those computers, which are used to solve variety of problems by changing the program or instruction. e.g., to make small database calculation, accounting, etc.

 Special Purpose Computer :  Special purpose computers are those computers which are used to solve a single and dedicated type of problem. e.g., automatic aircraft landing, multimedia computer, etc.