Random Access Memory (or RAM) is a type of computer memory that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes. This is a somewhat misleading definition, since both RAM and ROM allow random access. RAM should be referred to as read/write Memory, and ROM as read only. In common usage, the RAM is synonymous with 'Main Memory' or 'System Memory', which is the memory available to software, such as the Operating System.
RAM comes in many different forms, but two basic ones stand out: volatile, and non-volatile. From there, two types of RAM emerge: Static RAM (SRAM) and Dynamic RAM (DRAM); the latter has speed advantages, while the former has stability advantages. RAM is the most common type of memory found in computers, and other electronic devices, such as printers.
The two types differ in the technology they use to hold data; DRAM needs to be refreshed thousands of times per second, while SRAM does not, which makes it faster. However, SRAM is also more expensive. Both types of RAM are volatile, meaning that they lose their contents when the power is turned off.
DRAM is by far the most common, and is available in quite a few varieties. The earliest were called SIMMs. These were later extended to become EDO SIMMs.
The next major enhancement came in the form of DIMMs. If DIMMs were to be compared to SIMMs, DIMMs were essentially two SIMMs combined together.
Early SDRAM DIMMs, the most common RAM implementation in personal computing, were technically SDR, although this never became a differentiation until the advent of DDR RAM. DDR has since evolved into DDR2, DDR3, and DDR4. RAM has also been adapted for use by graphics card manufacturers; SGRAM was used first, followed by VRAM, and then latter by the GDDR series of RAM implementations, currently on GDDR5.