RAM- is best known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell. It also comes from hardware components wired into or attached to the motherboard the main circuit board of your computer. RAM is used to run certain basic programs and functions that your computer needs to operate correctly and functions only while the computer is receiving power. Programs you're using are written in RAM temporarily while the computer is processing them. Think of RAM as a playing field a large open area where your programs function. Each program takes up a certain amount of space: the field can accommodate one or several different programs at one time, but its capacity is limited. When you shut down a program, it disappears from RAM and ideally the space it occupied can be reused.
ROM- Read only memory is an acronym for read only memory a type of unchangeable memory residing in chips on your motherboard. ROM contains the bare minimum of instructions needed to start you computer. Because it's used for critical functions, it can't be removed short of ripping it out of the motherboard; adding to it is just as difficult. Think of it as analogous to municipal utilities, such as gas and electricity. If you want a different configuration, you'll have to "move on" to a different motherboard of computer. Incidentally the term "ROM" is also used, not entirely correctly, when referring to some kinds of storage media that can't be modified, such as CD-Rom's.
Virtual- Virtual memory is a software method of making a computer appear and act as though it has more RAM (Random Access Memory) than is actually present. By reallocating and using available disk space, the operating system of the machine transparently swaps chucks of data between a hard disk and physical memory. The advantage of doing this is that larger application programs can be loaded, more applications can be used simultaneously, and the programs have more space to store their data. A drawback of virtual memory is that it slows down system operation and is limited to the amount of disk space that is free on the system. Virtual memory is implemented on most major computer operating system including Unix, VMS, Macintosh System 7.x, Microsoft Windows, Windows NT, and OS/2
Caching- If you have been shopping for a computer then you have heard the word "cache". Modern computers have both L1 and L2 caches, and many now also have L3 cache. You may also have gotten advice on the topic from well-meaning friends, perhaps something like "Don't buy that Celeron chip, it doesn't have any cache in it!" It turns out that caching is an important computer -science process that appears on every computer in a variety of forms. There are memory caches, hardware and software disk caches, page caches and more. Virtual memory is even a form of caching.
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