Register, cache memory and main memory are internal memory. Magnetic Disk, removable media are external memory. Internal memories are semiconductor memory. Semiconductor memories are categoried as volatile memory and non-volatile memory.

RAM: Random Access Memories are volatile in nature. As soon as the computer is switched off, the contents of memory are also lost.

ROM: Read only memories are non volatile in nature. The storage is permanent, but it is read only memory. We can not store new information in ROM.

Several types of ROM are available:

• PROM: Programmable Read Only Memory; it can be programmed once as per user requirements.
                    
• EPROM: Erasable Programmable Read Only Memory; the contents of the memory can be erased and store new data into the memory. In this case, we have to erase whole information.
                                   
• EEPROM: Electrically Erasable Programmable Read Only Memory; in this type of memory the contents of a particular location can be changed without effecting the contents of other location.