The term "memory" can be used for anything in a computer that stores data, even temporarily. Several kinds of computer memory use different technologies. Though common usage has them playing to the best advantage of each, they can be confusingly interchangeable. Fundamentally, anything to which a computer can write information and later read it back counts as memory.
The Hard Disk Drive
A hard disk drive (hard drive or HDD for short) is the long-term memory of a computer. This is where you store documents, music, games and other files while you are not using them.
Internally, hard drives work like a record: there is a spinning platter, and an arm with a "head" that touches different parts of the platter to read from or write to them. On a record, information is stored in the peaks and troughs of a groove; on a hard drive information is stored in the polarity of tiny regions.
Hard disks can store massive amounts of data: the largest drives now store more than 2 terabytes. Unfortunately the design relies on mechanical motion, and therefore is relatively slow and prone to failure. A modern hard disk can access stored information at about 70 megabytes per second.
Random-Access Memory (RAM) is entirely electronic. The name refers to the fact that this kind of memory can access any information at any time, with equal speed. This is in contrast to memory such as tapes, which have to wind to information, or hard drives, which have to spin to it. RAM is the fastest kind of memory, and is used to feed information to the CPU "brain" of the computer. Programs or documents are stored in RAM only while they are being used, like the short-term memory in a human brain. In RAM, information is stored in the state of thousands of electronic circuits or capacitors. This structure allows RAM to access information at speeds up to 3,200 megabytes per second. The information disappears without a power supply however, making this "volatile" memory.
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Flash memory is a kind of Random-Access Memory that is non-volatile, that is to say it is stable even without power. Flash memory stores information in thousands of floating-gate transistors, each with a changeable resistance to electric current. Though not as fast as RAM, flash memory has the advantage of being incredibly durable. Memory cards can survive all sorts of rough handling, and even immersion in water. This makes flash memory perfectly suited for portable uses such as "thumb drives," camera memory and MP3 players. Flash memory is relatively slow; high speed flash memory can still only read 22 megabytes per second.
Many computers use tape as a storage medium. Tapes look and function just like audio cassettes: there is a spool of magnetic ribbon, and information is stored in the changing polarity of the ribbon. Tapes are used almost exclusively by large organizations for long-term, stable storage. While hard drives are prone to mechanical problems, and RAM needs a constant power supply, tapes can sit in a closet for years and retain the same information. The drawback is that a tape must spool (fast-forward or rewind) to the desired data before reading it, taking considerable time. Once the correct part of the ribbon has been found, tapes can transfer information just as quickly as any modern hard drive, however.
CDROM stands for Compact Disk Read-Only Memory, and it remains a popular format for portable storage. CDROM disks are read in much the same manner as a record (or a hard drive): they are spun, and a laser light reads the peaks and troughs in the aluminum surface of the disk. CDs have reasonable durability, but speeds are extremely slow compared with other kinds of memory. The fastest CDROM drives on the market read at less than 8 megabytes per second.
One of the most confusing things about computer memory is that it can be largely interchangeable. It is possible to use RAM as a hard drive, or a hard drive as portable storage, and in specialized situations this is even common. For example, some new laptops now use flash memory instead of a hard drive. These "solid state drives" are much more durable than traditional hard drives, but transfer speeds are slower. Many companies manufacture external hard drives, which really are hard disk drives in a case, acting as if they were flash memory.