I'm reasonably familiar with the virtual memory systems on classic UNIX, Solaris and current Linux. However, now that I'm even more a Mac user and (back) in the BSD world, I find myself trying to recall the specific details of memory states, active, inactive, free, wired.
Here's a good reference. It's not the most lucid discussion but it's accurate.
Chapter 7. Virtual Memory System, from the FreeBSD Architecture Handbook, The FreeBSD Documentation Project at freebsd.org.