Memory

RAM

RAM, short for random access memory, is a small stick inside your computer where all the programs currently in use are held. They are held here because accessing RAM is a lot faster than accessing the hard drive. In most computers there are two sticks of RAM, this is because it is faster to draw data from two sticks of RAM (or even four+) than one.

It should be noted that RAM will always run at the same frequency, so a faster stick of RAM would run as fast as the slower stick of ram if placed together. Below is a typical stick of RAM:

The two things to look for when considering RAM is capacity and frequency. The capacity of RAM is what should be considered first as frequency does not have much of an effect on performance. A typical set of RAM would be two sticks of 4gb ram running at 1600mhz.
If you want to "splash out" you can get more performance RAM like the one below:
This may be running at speeds like 2400mhz. It can do this because it has heatsinks which keep it cool.

ROM

ROM, read only memory, is memory that can only be read. ROM, in a computer normally contains the BIOS, which contains the boot program. As soon as the computer turns on it will look in the ROM to load the boot program. This will then start up the computer. ROM is usually very small, only around 2mb in size. ROM can be changed via a process called flashing. Flashing is slightly dangerous because if something goes wrong the ROM will be corrupted and the computer may not be able to start.

Virtual Memory

Virtual memory is a portion of your hard drive that is used as RAM when your RAM has been filled up, this allows you to keep your programs running normally and it keeps your computer running normally.

Flash

Flash memory is memory is like a hard drive in the fact it does not need to be constantly refreshed, but it is like RAM in the fact that the operating system and files can be stored there while in use. But, like a hard drive they can be stored there when not in use.

Unfortunately flash corrupts easily.