Random Access Memory (RAM) is the main memory of a computer. Main memory should not be confused with hard disk space which is used to permanently store data. Main memory is used to store all of the working information for the computer such as the operating system, user programs and data. RAM is volatile. The contents of RAM are lost when the computer is switched off.
Dynamic RAM (DRAM) is the most common type of RAM used in computers. It is relatively easy to manufacture and so is cheap. However, DRAM contents must be continually refreshed. The process of refreshing the memory takes time and while the memory is being refreshed it cannot be read from or written to. This makes DRAM cheap but slow.
Static RAM (SRAM) is physically different from dynamic RAM as the memory contents do not have to be continually refreshed. This means that it can always be accessed and so SRAM is a faster type of memory. However the process of manufacturing the memory and the components used make it more expensive. For this reason SRAM is used in smaller quantities where fast memory is required, such as cache.
For further information see the Wikipedia article on Random Access Memory.
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