Introduction Computer is an electronic data processing device which takes input as data and instruction and produces output as information. It makes work easy for the user. It can perform various Operations like calculations‚ data processing‚ giving instructions of specific task and many more that's why we will say it is very useful for the user. It is a programmable device means it executes task according to running the instructions by users. We have to give every instructions to the computer and that will give the complete output to uss. 

Like- We have to tell what type of data‚ how to perform different different calculations or where to store the complete data. The one and most important thing is the computer only understand the binary language. It uses only two digit 0 and 1‚ true or false.

The basic components of the computer - 

1. Central processing unit (CPU) 

2. Memory (RAM) 

3. Storage (hardware and software) 

4. Input and output devices. 

5. And several peripheral devices. 

Computer have following characteristics also -

1. It is an electronic data processing device. It consist of gates‚ transistors‚ diodes‚ I.C.’s etc.

2. Computers have only two operational states‚ represented by binary digits 0 and 1. It is also called digital computers.

3. Computers can store data and instructions can be executed directly without any personal involvement. 

4. They take decision according to the value of data and instructions.

5. They can perform arithmetic operations as well as logical operations.

6. It's working speed is fast. 

7. It's accuracy is very high and the degree of accuracy depends upon its design.

8. It is a versatile device.

Classification of Computers - 

• According to technology.

1. Analog computer. 

2. Digital computer.

3. Hybrid computer.

1. Analog computer:- Those computers which operates on continuously varying data‚ such as reading of temperature‚ speed of vehicle‚ pressure of a fluid etc is known as analog computer. 

2. Digital computer:- Those computers which operates on discrete values is known as digital computers. The discrete value are 0 & 1. The Universal Automatic Computer was the first digital computer which was not of a kind. 

3. Hybrid computer:- It is the combination of analog and digital computers. Data is input with the help of analog devices and then it is converted to digital signal with the help of analog to digital converter. 

• According to size. 

1. Micro computer.

2. Mini computer.

3. Super mini computer.

4. Mainframe computer.

5. Super computer. 

1. Micro computer :- A Micro processor combined with memory and interface modules is called a micro computer. The word ‘ micro’ is used to represent the small physical size of the components used in design. 

2. Mini computers :- Mini computers are faster than micro computers. The CPU Speed is around 1 Million instruction per second. They are more popular in laboratories and as processor in many special purpose system such as those used in image processing‚ type setting etc.

3. Super Mini computer :- Those are 32 bit word length machines and support memory sizes in the range 1 M byte to 8 M bytes. The CPU speeds is nearly 2 million instructions per second. A wide range of fast peripherals are supported by this machine. 

4. Mainframe computer :- Midicomputer also known as mainframe computers. It is the machines with word lengths of 32-64 bits. These computers normally support a memory size in the range 2 M bytes to 64 M bytes. The CPU speeds lies in the range 1 to 10 million instruction per second. 

5. Maxi or super computer :- Maxi-computers also called super-computers are the fastest computers ever made. It is used to solve a range of large scale problems which require extensive numeric computation. These computers are used in application in electronic‚ weather forecasting etc.