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Random Access Memory (RAM)
The kind of memory used for holding programs and data being executed is called random access memory or RAM. RAM differs from read-only memory (ROM) in that it can be both read and written. It is considered volatile storage because unlike ROM, the contents of RAM are lost when the power is turned off. RAM is also sometimes called read-write memory or RWM. This is actually a much more precise name, so of course it is hardly ever used. :^) It's a better name because calling RAM "random access" implies to some people that ROM isn't random access, which is not true. RAM is called "random access" because earlier read-write memories were sequential and did not allow random access. Sometimes old acronyms persist even when they don't make much sense any more (e.g., the "AT" in the old IBM AT stands for "advanced technology" :^) ).
Obviously, RAM needs to be writeable in order for it to do its job of holding programs and data that you are working on. The volatility of RAM also means that you risk losing what you are working on unless you save it frequently.
RAM is much faster than ROM is, due to the nature of how it stores information. This is why RAM is often used to shadow the BIOS ROM to improve performance when executing BIOS code. There are many different types of RAMs, including static RAM (SRAM) and many flavors of dynamic RAM (DRAM).
Next: Static RAM (SRAM)