The Windows swap file is used to augment the RAM. Essentially, it is a special place on the hard drive where items from memory can be temporarily stored for fast retrieval. For example, you might have five programs open at one time, but you are using only one at a time. If the system is running low on RAM, it can take the program or file that has been inactive the longest and move it to the swap file.

The Windows swap file used to end in a .swp extension; since Windows XP, however, it ends in pagefile.sys. It is typically found in the Windows root directory. The swap file is a binary file. Given that the swap file is used to augment RAM, it is often referred to as virtual memory.