OS X El Capitan: Quit apps
You don’t have to quit apps once you’re done working in them—OS X allows you to work with several apps open at once. Features such as Compressed Memory and App Nap keep your Mac running fast, and save power when many apps are open.
However, if you do want to quit an app, or make sure that apps don’t reopen when you restart your Mac, you can easily do that.
Quit an app
Set all app windows to close when quitting apps
By default, when you reopen an app, any windows that were open when you quit the app reopen. If you don’t want windows to reopen, you can set them to close when you quit.
Choose Apple menu > System Preferences, click General, then select “Close windows when quitting an app.”
Quit all apps when you restart, shut down, or log out
Choose Apple menu, then choose Restart, Shut Down, or Log Out. For more ways to restart, shut down, and log out, see Log out, sleep, restart, and shut down.
In the dialog that appears, deselect “Reopen windows when logging back in.”
If an app stops responding, you can force quit the app. Choose Apple menu > Force Quit, or press Command-Option-Esc, select the app, then click Force Quit. When you force an app to quit, you may lose unsaved changes.