Free: Free memory is not being used and is immediately available.
Wired: Wired memory contains information that must always stay in RAM.
Active: Active memory contains information that is actively being used.
Inactive: Inactive memory contains information that is not actively being used. Leaving this information in RAM is to your advantage if you (or a client of your computer) come back to it later.
Used: Used memory is being used by a process or by the system. It’s the sum of wired, active, and inactive memory. If the system requires memory, it takes free memory before used memory.
VM size: VM size is the total amount of virtual memory space reserved by the Mac OS X and your applications. The actual amount of virtual memory being used is likely to be much less, because Mac OS X and applications frequently reserve virtual memory space that they don’t use.
Page ins: The amount of information copied from your hard disk to RAM. The numbers in parentheses are the rate at which information is currently being copied.
Page outs: The amount of information copied from RAM to your hard disk. The number in parentheses is the rate at which the information is currently being copied. If your computer is performing its normal workload and that rate is above zero, adding RAM to your computer might improve performance.
Swap used: The amount of hard disk space currently being used as virtual memory.