Initially, the generation term was used to distinguish between varying hardware technologies. Nowadays, generation includes both hardware and software, which together make up an entire computer system. Computer generation refers to historical stages of computers as characterized by the various technologies. With each new generation, computer circuitry, size, and parts have been miniaturized, the processing and speed doubled, memory got larger, and usability and reliability improved.
The five computer generations are characterized by electrical current flowing through processing mechanisms listed below:
The first within vacuum tubes
The second within transistors
The third within integrated circuits
The fourth within microprocessor chips
The fifth unveiled smart devices capable of artificial intelligence.

The period of first generation was from 1946-1959. The computers of first generation used vacuum tubes as the basic components for memory and circuitry for CPU (Central Processing Unit). These tubes, like electric bulbs, produced a lot of heat and the installations used to fuse frequently. Therefore, they were very expensive and only large organizations were able to afford it. In this generation, mainly batch processing operating system was used. Punch cards, paper tape, and magnetic tape was used as input and output devices. The computers in this generation used machine code as the programming language.

The main features of the first generation are −

Vacuum tube technology
Unreliable
Supported machine language only
Very costly
Generated a lot of heat
Slow input and output devices
Huge size
Need of AC
Non-portable
Consumed a lot of electricity

Some computers of this generation were −

ENIAC
EDVAC
UNIVAC
IBM-701
IBM-650

The period of second generation was from 1959-1965. In this generation, transistors were used that were cheaper, consumed less power, more compact in size, more reliable and faster than the first generation machines made of vacuum tubes. In this generation, magnetic cores were used as the primary memory and magnetic tape and magnetic disks as secondary storage devices. In this generation, assembly language and high-level programming languages like FORTRAN, COBOL were used. The computers used batch processing and multiprogramming operating system.

The main features of second generation are −

Use of transistors
Reliable in comparison to first generation computers
Smaller size as compared to first generation computers
Generated less heat as compared to first generation computers
Consumed less electricity as compared to first generation computers
Faster than first generation computers
Still very costly
AC required
Supported machine and assembly languages

Some computers of this generation were −

IBM 1620
IBM 7094
CDC 1604
CDC 3600
UNIVAC 1108

The period of third generation was from 1965-1971. The computers of third generation used Integrated Circuits (ICs) in place of transistors. A single IC has many transistors, resistors, and capacitors along with the associated circuitry. The IC was invented by Jack Kilby. This development made computers smaller in size, reliable, and efficient. In this generation remote processing, time-sharing, multiprogramming operating system were used. High-level languages (FORTRAN-II TO IV, COBOL, PASCAL PL/1, BASIC, ALGOL-68 etc.) were used during this generation.

The main features of third generation are −

IC used
More reliable in comparison to previous two generations
Smaller size
Generated less heat
Faster
Lesser maintenance
Costly
AC required
Consumed lesser electricity
Supported high-level language

Some computers of this generation were −
IBM-360 series
Honeywell-6000 series
PDP (Personal Data Processor)
IBM-370/168
TDC-316

The period of fourth generation was from 1971-1980. Computers of fourth generation used Very Large Scale Integrated (VLSI) circuits. VLSI circuits having about 5000 transistors and other circuit elements with their associated circuits on a single chip made it possible to have microcomputers of fourth generation. Fourth generation computers became more powerful, compact, reliable, and affordable. As a result, it gave rise to Personal Computer (PC) revolution. In this generation, time sharing, real time networks, distributed operating system were used. All the high-level languages like C, C++, DBASE etc., were used in this generation.

The main features of fourth generation are −
VLSI technology used
Very cheap
Portable and reliable
Use of PCs
Very small size
Pipeline processing
No AC required
Concept of internet was introduced
Great developments in the fields of networks
Computers became easily available
Some computers of this generation were −
DEC 10
STAR 1000
PDP 11
CRAY-1(Super Computer)
CRAY-X-MP(Super Computer)

The period of fifth generation is 1980-till date. In the fifth generation, VLSI technology became ULSI (Ultra Large Scale Integration) technology, resulting in the production of microprocessor chips having ten million electronic components. This generation is based on parallel processing hardware and AI (Artificial Intelligence) software. AI is an emerging branch in computer science, which interprets the means and method of making computers think like human beings. All the high-level languages like C and C++, Java, .Net etc., are used in this generation.

The main features of fifth generation are −
ULSI technology
Development of true artificial intelligence
Development of Natural language processing
Advancement in Parallel Processing
Advancement in Superconductor technology
More user-friendly interfaces with multimedia features
Availability of very powerful and compact computers at cheaper rates
Some computers of this generation were −
Desktop
Laptop
NoteBook
UltraBook
ChromeBook