The total amount of RAM (Random Access Memory) installed might not always be sufficient to run all the applications on the system. Windows memory management implements and manages what is called Virtual Memory. It’s the method of extending the available physical memory on a computer. PageFile is a file on disk which is used to swap physical memory pages to and from a disk. Virtual Memory comprises of total available physical memory(RAM) on the computer and size of the PageFile on disk.
Virtual Memory Manager
Virtual Memory Manager is responsible for mapping physical memory and virtual address spaces for a process. It keeps track of each mapping for VAS addresses using a page table, and the mapping information itself is stored in a Page Table Entry (PTE). The PTE is 4K in size and shares the same space as other system information and structures of the system.
Most recently referenced pages are located in RAM. Those which aren't recently referenced for a while may be written to the PageFile. This is called swapping or paging out memory.
Thus there is no gaurantee that once a particular page was accessed from certain location within memory, the same page is available at the same location the next time its accessed. The next time it may be loaded from the PageFile on disk so the request will be slower. This is known as a Page Fault and it occurs without users intervention or knowledge.The process of loading the page back from the PageFile into physical memory, also called swapping or paging in memory.
/3GB switch in 32-bit systems