You can quit most applications on your Mac using the app's main menu. When an app freezes or is otherwise unresponsive, take advantage of the Force Quit feature on your Mac.
Force Quit in the Force Quit Window
The Force Quit window displays a list of applications currently running on your Mac. The quickest way to load the Force Quit window is to press "Command-Option-Esc" on your keyboard. You can also load the window by clicking the "Apple" menu in the top-left corner of your screen and selecting "Force Quit." Once the Force Quit window is open, select the problematic app and click the "Force Quit" button.
Force Quit in Activity Monitor or the Dock
You can also force an app to quit using the Activity Monitor. Click the "Activity Monitor" icon in your Applications folder, select the name of the application you want to quit and then click the "Force Quit" button in the upper-left corner of Activity Monitor. The Force Quit button resembles a stop sign with an X on it.
The Force Quit feature is also available directly via your Mac's Dock. Press "Command-Option-Esc" on your keyboard, click the Dock icon corresponding to the app you want to quit and select "Force Quit" on the pop-up menu.
Consequences of Forcing an App to Quit
Forcing an application to quit may result in the loss of any unsaved data currently open in that app. If you haven't saved recent changes to a file in Microsoft Word, for example, those changes may not be present in the document the next time you open it. Similarly, if you force-quit a Web browser you may lose open tabs.