Activity Monitor by default shows a "memory" column. I always thought that it's numbers seem a bit low. I just discovered on Macrumors (thanks W0lf) that there is another column called "real memory" that uses like 2x as much in some instances (see screenshot). Which one is actual usage? What's the difference? Also what is "private memory" which is another option. Thanks, I'm just trying to learn more about computers.