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 stored in virtual memory, it is loaded into the actual memory address.
The picture shows the Virtual Memory settings window in Microsoft Windows. From this window, you can view information including total paging file size across all drives. You can also enable and disable the paging file, and define a custom size of the page file.