The different columns in Activity Monitor are a little confusing
Taken from my answer at Server Fault:
Mac OS X Memory Jargon:
Wired : This refers to kernel code and such. Memory that should not ever be moved out of the RAM. Also know as resident memory.
Shared : Memory that is shared between two or more processes. Both processes would show this amount of memory so it can be a bit misleading as to how much memory is actually in use.
Real : This is the "real" memory usage for an application as reported by task_info() - a rough count of the number of physical pages that the current process has. (RSIZE)
Private : This is memory that a process is using solely on it's own that is used in Resident memory. (RPRVT)
Virtual : The total amount of address space in the process that's mapped to anything - whether that's an arbitrarily large space for variables or anything - it does not equate to actual VM use. (VSIZE)
Active : Memory currently labelled as active and is used RAM.
Inactive : "Inactive memory is no longer being used and has been cached to disk. It will remain in RAM until another application needs the space. Leaving this information in RAM is to your advantage if you (or a client of your computer) come back to it later." - Mac OS X Help
Free : The amount of RAM actually available without any data.
The best documentation I know of (and have been able to find in followup research) is Apple's own Managing Memory article on their developer website.
Apple updated the Activity Monitor in OSX 10.9 (Mavericks). They have a good article about what everything means here http://support.apple.com/kb/HT5890. In summary...
Memory Pressure graph
The combination of Free, Wired, Active, Inactive & Used memory statistics in previous versions of Activity Monitor have been replaced in Mavericks with an easy to read "Memory Pressure" graph.
Memory pressure is indicated by color:
Real mem relates to physical memory (actual RAM modules in your computer). Virtual Mem is how much "fake" memory is allocated to the process, meaning memory that is allocated on the permanent storage medium (hard drive, solid state drive, etc) for that process. Shared memory is physical (Real) memory that can be shared with other processes. Private memory is "real" memory that can only be used by the process it is allocated to.
These explanations may help as well... directly from activity monitor --> help --> viewing system memory usage:
Here is an explanation of some of the information displayed at the bottom of the memory pane: