Physical Memory usage
Physical Memory usage will normally remain close to the total amount of physical memory installed on the system unless the amount of physical memory you have exceeds the amount of virtual memory that Windows is using.
Windows will normally keep some physical memory available (free) for immediate reuse.
Virtual Memory usage
Virtual Memory usage will increase and decrease as Windows processes request and give up memory and as processes are started and stopped.
A steady increase in virtual memory usage can indicate that a process on the system has a memory leak.