A digital computer in its simplest form is a fast electronic calculating machine that accepts digitized information from the user, processes it according to a sequence of instructions stored in the internal storage, and provides the processed information to the user. The sequence of instructions stored in the internal storage is called computer program and internal storage is called computer memory.

According to the size, cost computational power and application computers are classified as:

1. Microcomputers

2. Minicomputers

3. Desktop computers

4. Personal computers

5. Portable computers

6. Workstations

7. Mainframes or Enterprise systems

8. Servers

9. Super computers

1. Micro computers: As the name implies micro-computers are smaller computers contains only one Central Processing Unit. One distinguishing features of Microcomputer is that the CPU is usually a single integrated circuit called Microprocessor. Microcomputer is the integration of microprocessor and supporting peripheral (memory and I/O devices). The word length depends on the microprocessor used and is in the range of 8 bits to 32 bits. These types of computers are used for small industrial control, process control and where storage and speed requirements moderate.

2. Minicomputer: Minicomputers are the scaled up version of the microcomputer with the moderate speed and storage capacity. These are designed to process smaller data words, typically 32-bit words. This type of computers is used for scientific calculation, research, data processing application and many others.

3. Desktop computers: These computers are the computer which is usually found on a home or office desk. They consist of processing unit, storage unit, visual display an audio as output units, and keyboard and mouse as input units. Usually storage unit of such computer consists of hard disks, CD-ROM, and diskettes.

4. Personal computers: The personal computers are the most common form desktops computers.. They found wide use in homes, schools and business offices.

5. Portable Notebook computer: Portable notebook computer are the compact version of personal computers. The laptop computers are the good examples portable notebook computers

6. Workstations: Workstations have higher computation power than personal computers. They have high resolution graphics terminals and improved input/output capabilities. Workstations are used in engineering applications and in interactive graphics applications.

7. Mainframes or Enterprise systems: Mainframe computers are implemented using two or more central processing units (CPU). These are designed to work at very high speeds with large data word lengths, typically 64 bits or greater. The data storage capacity of these computers is very high. This type of computers are used for complex scientific calculations, large data processing applications, military defence control and for complex graphics applications (e.g. : For creating walk through with the help of animation software).

8. Servers: These computers have large storage unit and faster communication links. The large storage unit allows storing sizable database and fast communication links allow faster communication of data blocks with computers connected in the network. This computer serves major role in internet communication.

9. Supercomputers: These computers are basically multiprocessors computers used for the large-scale numerical calculations required in applications such as weather forecasting, robotics engineering, aircraft design and simulation.