###[Reading system memory usage in Activity Monitor](http://support.apple.com/kb/ht1342)

from support.apple.com gives a detailed explanation about the different "types" of RAM.

* **Free memory:** This is RAM that's not being used.
 
* **Wired memory:** Information in this memory can't be moved to the hard disk, so it must stay in RAM. The amount of Wired memory depends on the applications you are using.
 
* **Active memory:** This information is currently in memory, and has been recently used.
 
* **Inactive memory:** This information in memory is not actively being used, but was recently used.

* **Used:** This is the total amount of memory used.