Activity Monitor helps me manage my applications. Let me guide you in a very simple manner on how to use Activity Monitor?
Activity Monitor is located inside Utilities folder, the same folder as Terminal. The full path is Macintosh HD → Applications → Utilities → Activity Monitor.
It's recommended to set the Activity Monitor update frequency to less often. It will reduce the CPU Usage of Activity Monitor. You can try to compare the CPU Usage when Activity Monitor is set to very often and less often. This can be set at Menu Bar → View → Update Frequency → Less often.
By sorting by CPU Usage, it's easier for us to monitor our Applications. Besides, it will help us spot not responding Application. Not responding Application name will be shown in red. Clicking on column labeled %CPU will have Activity Monitor does the sort for you.
To my surprise, my Quicksilver %CPU suddenly jumped to 85.6%. As you can see in my snapshot. Since I didn't know what happen, I chose to quit it.

Kill Application can be done by highlighting the Application and hit the Quit Process button located at top-left corner of Activity Monitor pane. However, you have to make sure you don't kill the innocence. These following Application can be considered guilty and should be killed.
Floating window can help us see our CPU activity when the Activity Monitor is hidden. In order to make this floating window shows up, choose Show Horizontal Floating Window. It's Menu Bar → Window → Show Floating CPU Window → Horizontally or with keystrokes Command-4.
This floating window can be moved. I prefer horizontal floating window for my Activity Monitor because I can attach it into my menu bar.
Hit keystrokes Command-W or click on the cross button on the top-left corner of Activity Monitor to quit it. Don't use Command-Q if you still want to use the floating window.
To show up the activity monitor, click on its icon on the Dock and press Command-1.
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