Virtual memory is a method of using the computer hard drive to provide extra memory for the computer. Segments of memory are stored on the hard drive known as pages. When a segment of memory is requested that is not in memory it is moved from the virtual memory to an actual memory address.
In the picture to the right is the Virtual Memory settings window in Microsoft Windows. From this window you can view information such as the total paging file size for all drives, enable and disable the paging file, and define a custom size of the page file.