- Physical Memory: The amount of RAM installed.
- Memory Used: The amount of RAM being used and not immediately available.
- Virtual Memory: The amount of disk or flash drive space being used as virtual memory.
- Swap Used: The space on your drive being used to swap unused files to and from RAM.
- App Memory: The amount of space being used by apps.
- Wired Memory: Memory that can’t be cached to disk, so it must stay in RAM. This memory can’t be borrowed by other apps.
- Compressed: The amount of memory in RAM that is compressed.
- File Cache: The space being used to temporarily store files that are not currently being used.
The combination of Free, Wired, Active, Inactive and Used memory statistics in previous versions of OS X have been replaced in Mavericks with "Memory Pressure", that can be monitored in Memory Cleaner with a gauge. Read more at OS X Mavericks: About Activity Monitor.