RAM
RAM is a storage unit using semiconductor devices, which is used for main memory of computers. Main memory is also sometimes called RAM.
RAM can operate so fast and be accessed directly from CPU (Central Processing Unit) because RAM reads and writes data electrically, but the cost per unit amount is so high that it cannot be used for large amount.
Because of this, external storage units (auxiliary storage units) like hard disks or floppy disks are installed in computers other than RAM. When users start programs and process data, only necessary information are used by RAM, and external storage units are used to save data for long-term.
The storage unit whose content cannot be changed once it is used is called ROM, and it is used to record programs used when the power is on at first.