There are three general kinds of computer memory. One is called "Random Access Memory" and is known better by its acronym, RAM. RAM is temporary storage. All of the data in RAM is erased if the computer is powered off.

The second kind of memory is called Read-Only Memory, or ROM. Many people might recognize the acronym for read-only memory, since it was part of a storage format called "CD-ROM" which stood for Compact Disc Read-Only Memory. There are two major differences between RAM and ROM. One is that ROM retains its data even if the computer is turned off. This makes it possible, for example, to distribute software on compact discs and DVDs. The second is that once read-only memory has been written to, it cannot be changed or updated.

The third kind of computer memory is simply known as "mass storage." Mass storage combines the advantages of RAM and ROM. Mass storage both retains its data and it can be changed and updated at the same time. Mass storage might include such devices as hard drives, floppy disks, tape drives, USB Flash drives, various kinds of firmware devices, E-PROMs and so forth.

Computer memory is measured in bytes. Each byte consists of eight bits. Each bit is simply a switch that can be on or off. On is a value of one. Off is a value of zero. Which of the eight bits are on, or ones, and which are off, or zeros, determines the value of the byte, expressed in binary math. It sounds complicated, but it really is quite simple. Essentially, each letter in this sentence consists of one byte of memory. Each different letter has a different pattern of ones and zeros in its byte of memory.

Decades ago, the upper limit of computer memory was measured in kilobytes, or 1024 bytes of storage. Capacities eventually increased to megabytes, consisting of 1024 kilobytes, then to gigabytes, consisting of 1024 megabytes, and so on. There are also terabytes (1024 gigabytes), petabytes (1024 terabytes) and even exabytes (1024 petabytes, or one billion billion bytes of data).

It is interesting to note that some of the first commercially available personal computers had far less than 1/1000 of a gigabyte of RAM, and had to rely on floppy disks for mass storage, some with less than 1/2000 the capacity of a CD-ROM. Before that, computers had to rely on paper punchcards to store data, with great stacks of them being fed into a card reader in order to restore a program to the computer's active memory.

Currently the upper limit on computer RAM is usually expressed in a quantity of gigabytes, or billions of "letters" of memory. Hard drive capacities are also currently generally measured in hundreds of gigabytes, although the first terabyte hard drives, each with a capacity exceeding one trillion "letters" of memory, are likely just now becoming available commercially.

Most computers now have around one gigabyte of RAM and mass storage consisting of 100-200 gigabyte hard drives, plus a DVD drive and USB ports for flash memory. Once the terminology is understood, however, it is much easier to make better purchasing decisions when buying a new computer.