On Mac computers, the Activity Monitor application is equivalent to the Task Manager program found on Windows PCs. You can open Activity Monitor using either the Launchpad or the Applications folder on Macs running OS X Yosemite.
Use Launchpad to Open Activity Monitor
Step 1: Click the Launchpad icon on your Mac’s Dock to open Launchpad. You can also click the Launchpad icon in your Applications folder to open Launchpad.
Step 2: Select the Other group on the list of Launchpad icons.
Step 3: Click the Activity Monitor icon to open Activity Monitor.
Use Applications Folder to Open Activity Monitor
Step 1: Click the Applications folder on the Dock.
Step 2: Select the Utilities folder.
Step 3: Click the Activity Monitor icon to open the application.
Using Activity Monitor
Activity Monitor enables you to keep tabs on all programs and processes currently active on your Mac. Click one of the tabs along the top of the window to sort processes based on the contents of that column. Click CPU, for example, to sort all processes based on which ones are placing the highest demand on your Mac's processor. Similarly, click Memory to see which processes are using the most RAM. A higher number indicates an app or process that’s using more system resources. The higher the number is, the more likely it's slowing down your Mac. If an app or process is using too many system resources and won't quit normally, use Activity Monitor to force it to quit.
Using Activity Monitor to Force-Quit an Application or Process
One of the more common uses of Activity Monitor is to troubleshoot and potentially quit a problematic application. If an app or process is using too much memory or percent of the CPU, or if the app won't close properly, use the Force Quit command in Activity Monitor to make it quit.
Select the name of the app or process you want to quit and then press the Force Quit button in the top-left corner of the window.