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Let's start with "Page file" usage.
This is the amount allocated to the system page file. The page file is essentially a file used to store 'parts' of your physical memory on your HD, when you run out. Ever notice the lag time when you have, say, 10 applications open, and you switch over to an application which was idle for the past hour? This lag time is a result of the system swapping old data from the page file, back to physical memory.
So to sum it up, idle areas of the RAM are generally written to the page file, to make room for the more recent and frequently used data. This is the "page pool" area. There are, however, areas of the RAM that are crucial to the OS, and cannot be written out to the page file. These area's are called, "Non-paged" area's.
Read up on "Virtual memory" for more information.
Thanks to megatron @ vbforums