finally found a good explanation for the Mac specific terms Wired, Active, Inactive, and Free for the physical memory (RAM). You can digg it up here.
Just in short:
Wired memory is used by the OS and is pretty much untouchable. Another application can’t “borrow” wired memory.
Active memory is what is currently in use by running applications. Note that thanks to the splendors of virtual memory, all of the memory needed by an application isn’t necessarily contained here. If you look at a running process in the Activity Monitor list, you’ll see a column for Real and a column for Virtual Memory. Since we are talking about the amount of RAM in use, we won’t worry about virtual memory for the moment. If there is no inactive, or free memory, active memory can be used by other applications, but this causes the OS to write the current state of the active memory being traded to its owner’s virtual memory pages on disk before granting the memory to another application.
Inactive memory is memory that has recently been used by an application that is no longer running. OSX keeps track of what this is and what it belonged to because of the idea of temporal locality, the idea being that if you opened an application you are somewhat likely to do so again and if the memory is still labeled, the application can start very quickly. In the absence of sufficient free memory, inactive memory will be reclaimed by another running application that needs memory.
Free memory is just that, free. Nothing has a claim on it, and it’s up for grabs for any application that needs it.
As you can see from my screen-shot image above, I tend to run with around 100-200 Mb free, between 150 and 300 Mb wired, and the rest split between active and inactive. What does this memory labeling have to do with how fast or slow the machine runs? Simple. When you log in, OSX claims the memory it needs to do all of its chores. This is wired. Other applications claim a smallish chunk of active memory as they are opened. Most applications that need to keep track of any kind of history or user data gradually use up more memory the longer they are open. Every now and then an application will need to use some part of its memory that it doesn’t use frequently and OSX has to get this from disk and place it in that application’s active memory allocation. All of these things can slow down a system. Also, virtualization applications such as Parallels or VMWare’s Fusion require a large chunk of memory for the virtual machine that gets marked as wired.
More at the link specified above – Thank You.