Virtual Memory is a feature of an operating system that enables a process to use a memory (RAM) address space that is independent of other processes running in the same system, and use a space that is larger than the actual amount of RAM present, temporarily relegating some contents from RAM to a disk, with little or no overhead.
ROM – Read-only memory: On ROM, data is prerecorded for read only which can not be removed. ROM is nonvolatile and it retains its contents regardless the computer is on or off. Most personal computers contain a small amount of ROM that stores critical programs such as the program that boots the computer. In addition, ROMs are used in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs. There are a few variations of ROM such as programmable read-only memory(PROM), which is manufactured as blank chips on which data can be written with a special device called a PROM programmer.
RAM – Random access memory: whose contents can be accessed (read, write and remove) in any order. This is in contrast to sequential memory devices such as magnetic tapes, discs and drums, in which the mechanical movement of the storage medium forces the computer to access data in a fixed order. RAM is usually used for primary storage in computers to hold active information such as data and programs. Common forms of RAM are: SRAM (Static RAM) and DRAM (Dynamic RAM).