From Unofficial BOINC Wiki
The hardware component that serves as the Central Processing Unit's memory store that has real addresses that can be mapped into the Virtual Memory's address space. So, one moment the Physical Memory may have information from one part of the Virtual Memory space, the next moment it will have the contents from another location.
The process of moving programs and data into and out of the Physical Memory is called Swapping and is under control of the portion of the Operating System called, for reasons that escape me, the Virtual Memory Manager. Weird isn't it?
At any rate this is one of the many management tasks that is handled by the Operating System.