Hopefully, most of you have seen the Activity Monitor in OS X before. If not, you may want to check it out (go to Applications > Utilities > Activity Monitor). This is a great place to go to check on network traffic, memory usage, disk activity and much more. I don't spend a lot of time here, but often enough that I have a good understanding of what I expect to see, and it helps me to understand why things sometimes don't run as efficiently as it should.
One of the areas that I frequent in this utility is the "System Memory" tab. This shows me an overview of how my RAM is being used, as well as how much each application is using. You'll notice there is a pie chart that visually shows you the breakdown of how your RAM is being allocated. For most, this may seem meaningless. I want to help you understand the four categories to help you better understand your Mac. Here they are:
This is the amount of RAM that has not been used since the last time you turned your computer on. It's available for immediate use by any application.
Applications that use "wired" memory are those that are critical and cannot be offloaded to your hard disk (or "paged"). Examples of this are OS-related apps that control the stability and functionality of your experience. This RAM is not available for anything else.
This category of RAM is the most common. This is where most of your active applications will live, until it's either released or offloaded to the hard disk as a page file.
This is RAM that was once used, but is now available for another application to use.