While we Mac users use web browsers or email or listen to iTunes or use programs that make us happy or make money, the Mac is dedicated to keeping us happy and we rarely pay much attention to what happens inside the Mac to something goes a little wonky.
Here are two ways to see Mac's system status in real time; one is free and built into OS X, the other costs a few dollars, but is easier to see and use.
First on my list is a practical tool called Translucent. What you get is system information spread over a number of transparent floating windows instead of a single monolithic app window that can show system data you don't need.
Transparent displays more than enough system data for the next of Mac users. 
What I like most about Translucent is the amount of system data and how it can be customized to show what you need and waste what you don't. The app lives in the Mac menu, so the details are just a click away, but system status is visible in the line.
What you get is a quick view of Mac's CPU usage, RAM, network activity (both up and down), disk storage capacity and availability, and plenty of statistics to massage your curiosity gene.
Second, if a few dollars for Translucent is too much, OS X comes with a built-in utility that also monitors various internal systems. It is the Activity Monitor app in the Tools folder in the Applications folder.
Activity Monitor will not display the same system data as simple and easy as Reviewing other Mac monitoring tools, but it is priced properly. Getting to the same system data takes several clicks, but it's easy enough to see which apps use the Mac's CPU most, hog memory, take up space, and there's a new Energy setting that shows which apps have the most impact Mac power or battery.
Anyway, Translucent or Activity Monitor, you can see more of what's happening under the Mac cap.