Memory is the internal storage areas within computing systems. The computer memory is any physical device that can temporarily store data such as RAM (random access memory) or ROM (read only memory). Some computers also make use of virtual memory that expands the physical memory via the hard disk. Memory devices operate through integrated circuits and are used by operating systems, software and hardware.
Memory can be classified as temporary or permanent. Temporary memory does not store data when the computer or the hardware device is no longer powered. Computer RAM is an example of temporary memory. This is why if the computer freezes or restarts while you are working, data that has not been saved might be lost. Permanent memory, sometimes abbreviated as NVRAM, is a type of memory that stores the data even in the case of a loss of power. EPROM is another example for permanent memory. Virtual memory is the memory management capability of an operating system that uses hardware and software to temporarily transfer data from the random access memory (RAM) to the hard disk in order to compensate for insufficient physical memory. Virtual address spaces use active memory of the RAM and the inactive memory of the hard disk drives (HDD) to make available addresses that are contiguous that can store both the application and the data.
Memory is usually categorized as cache, primary and secondary:
Cache memory is very high speed and semiconductor based, it can improve the performance of the central processing unit (CPU). It serves as a buffer between the CPU and the primary memory. It is used to store data and application instructions most frequently requested by the CPU. Operating systems transfer data and parts of an application from the disk to the cache where the CPU can access them.
Primary memory (main memory) only stores data and instructions that are currently used by the computer. It has a limited capacity and data is lost when the power supply is switched off. It is usually comprised of a semiconductor device. Data and instructions that need to be processed are stored in the main memory. It has two sub-categories as RAM and ROM.
Secondary memory is known as external memory as well. CPU does not access this memory directly, instead it uses input/output channels for indirect access. The data stored on secondary memory is first transferred to the main memory, upon which it can be accessed by the CPU. CD-ROM, DVD and similar devices are examples of secondary memory.