This article describes the meaning of the Memory Active and Memory Usage indicators on the Performance tab in VirtualCenter.

Used memory

Used memory (more commonly named consumed memory) is the amount of memory used on the ESXi in total. This includes memory used by the VMKernel, other vSphere Services and Management agents, plus the the total consumed memory for all running virtual machines.

The latter part is however not an actual sum of what each of the guest operating systems in these virtual machines are consuming internally. 

ESXi employs a number of memory reclamation methods, such as transparent page sharing, where similar memory pages are shared among multiple VMs (Inter-VM TPS), or even deduplicated for this virtual machine (Intra-VM TPS) to keep its overall memory usage low and to allow for memory over-commitment. A one to one comparison between the memory used on ESXi with the values within each guest operating system is therefore hardly if not at all possible.
 

Active memory

Active memory on the other hand is that part of the used/consumed memory, which has been actively (hence the name) touched by I/O operations within the last 20 seconds.
Operating systems usually are not just utilizing memory for current operations, but also - by means of caching or pre-fetching - to to keep data available they estimate to be required later or regularly. This data can reside in memory for longer amounts of time without being touched, and therefore is not included in the active memory.

Used memory in the vCenter performance charts is usually displayed as a percentage value on how much of the available memory has been used. Active memory on the other hand is usually shown in kilobytes.

For more information, please refer to the vSphere Resource Management Guide .