Each application that runs on your Mac eats up system resources that your system could use to make other applications run more effectively. Identifying the currently running applications can help you to determine the applications you can shut down. You can check for currently running applications in several ways, depending on what you need to accomplish.
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View the Dock for any application icons with indicator lights beneath them. This indicates a currently running application. Click the icon to bring it in front of all other applications.
Press "Command-Option-Escape" to activate the Force Quit Applications dialog box. This window shows all the currently running applications, and it gives you a way to close an application that has stopped responding by selecting the application and clicking the "Force Quit" button.
Click the "Go" menu from the Apple menu bar, select "Utilities" and choose "Activity Monitor" to launch the application. Look in the Process Name field in the default CPU tab to view a list of all currently active applications and processes. You can click the various tabs to sort the items by CPU usage, memory usage and other attributes.