Each process in Windows gets a virtual address space.
Process -> Virtual Page -> Physical Page (via a page map).
This virtual address space is backed by the combination of physical memory and page file(s). So the virtual address space can be larger than the total amount of physical memory in the system.
The portion of a process's virtual memory that resides in physical memory is the processes working set.
When a thread attempts to use (not just requests) more physical memory than the system has available then Windows frees up memory by moving some pages out of physical memory (into the page file).