What is "virtual memory"?

What is virtual memory and why does my computer keep saying that it is increasing it? How can I keep this from happening all the time?

Need to increase your memory size (RAM):

Memory is hardware that your computer uses to load the operating system and run programs. It consists of one or more RAM chips that each have several memory modules. The amount of real memory in a computer is limited to the amount of RAM installed. Common memory sizes are 256MB, 512MB, and 1GB.

Because your computer has a finite amount of RAM, it is possible to run out of memory when too many programs are running at one time. This is where virtual memory comes in. Virtual memory increases the available memory your computer has by enlarging the address space, or places in memory where data can be stored. It does this by using hard disk space for additional memory allocation. However, since the hard drive is much slower than the RAM, data stored in virtual memory must be mapped back to real memory in order to be used.

The process of mapping data back and forth between the hard drive and the RAM takes longer than accessing it directly from the memory. This means that the more virtual memory is used, the more it will slow your computer down. While virtual memory enables your computer to run more programs than it could otherwise, it is best to have as much physical memory as possible. This allows your computer to run most programs directly from the RAM, avoiding the need to use virtual memory. Having more RAM means your computer works less, making it a faster, happier machine.