Computer has been modifying from the beginning to the end. There are many change has been occur in this process. As we know that there is change occur when time is running. So we can classify computer into different generation. The memory unit, processing capabilities and their mode of operation has help us to classify the computer into different generation.
 
 The development of different types of memory devices has given a sequence of development of computers from mainframe to microcomputer. Early computer were only the mainframe computer which were owned by the universities or government agency. The computer was costly and changing a program needed change in the cable connection also. The early computers used vacuum tubes. The vacuum tubes or valves were invented by Lee De Forest in 1908. The vacuum tubes were used as memory devices in the first generation computers.
 The computers used different kinds of memory devices. According to the use of memory devices the different generation are as follows
1. First generation computer
2. Second generation computers
3. Third generation computers 
4. Fourth generation computers
5. Fifth generation computers