SSD

SSD is very fast. SSD's are very good to boot up your operating system because it loads up everything much quicker than a HDD
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HDD
HDD's are very cheap and reliable but also they hold huge amounts of capacity.
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Flash Memory
Flash Memory is very expensive and is very small you can get fair amount of memory on them but also their portable USB sticks.
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RAM is essential to computers. RAM is used for the operating system but also its used as short term memory. There is two types of RAM dynamic and static. Dynamic RAM constantly refreshes because it forgets things so it refreshes to keep the information but the information is gone when the computer is turned off.