I just received a great PDF on memory statistic defintions for VirtualCenter:

It is important to note that some definitions refer to guest physical memory while others talk about machine memory. “Guest physical memory” is really virtual-hardware “physical” memory presented to the VM. “Machine memory” is the actual physical RAM in the ESX host. Each definition below will explicitly specify the type of memory to which it applies. See [2] pages 139 – 141 for more information about guest physical memory versus machine memory.

This distinction plays out in interesting and potentially confusing ways. The final section, “Differences between Memory Granted/Consumed and Memory Shared/Shared Common”, expands on the differences between guest physical memory and machine memory metrics.

Read this document, it really contains a lot of useful information!!!