The page file is one of those pieces of the operating system that administrators know that they need to have - but they can't always explain why they need it, or how to accurately size it. Since Windows 95, Windows-based operating systems have used a special file that acts as a sort of "scratch pad" to store modified pages that are still in use by some process. Page file space is reserved when the pages are initially committed, however the page file locations are not chosen until the page is written to disk. So, in simplistic terms, the page file is used by Windows to hold temporary data which is swapped in and out of physical memory in order to provide a larger virtual memory set.
When the system boots up, the Session Manager process determines the list of page files to open by reading the value in the HKLM\System\CurrentControlSet\Control\Session Manager\Memory Management\PagingFiles. This value contains the name of the paging file as well as the minimum and maximum size of each paging file. Windows supports up to 16 page files. On a 32-bit system running the normal kernel, the maximum size of each page file is 4095 MB. On x64 systems and x86 systems with the PAE kernel, the maximum page file size is 16 terabytes (16TB). On IA-64 systems, each page file can be as large as 32 terabytes.
To view the list of page files, you can either look at the HKLM\System\CurrentControlSet\Control\Session Manager\Memory Management\PagingFiles registry value. The paging file configuration settings are managed through the System utility in Control Panel. Below are two examples of page file settings: