Page faults generally refer to the process of retrieving items from memory that were placed into the virtual memory on the hard disk--aka the paging file.
As applications use memory, they may require more than the amount of physical RAM in the system. When this happens, Windows moves data that has not been recently accessed from physical RAM to the paging file on the hard disk. The next time an application requires that memory, a page fault occurs, and it must be retrieved from the hard disk.
Page faults are normal and will always occur. If they are excessive, they can have a negative impact on performance. Page faults can be measured in Performance Monitor, and give you some idea of whether a system needs more RAM.
This was first published in October 2001