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Your Mac will prevent you from shutting down if there is any application open in background, like iPhoto, iTunes etc. This is done to protect your files, so you won't lose any unsaved documents if you hit Shutdown accidentally.
To see what applications are preventing from Shutdown, just press Command, Option and Esc keys at same time or select Force Quit from Apple menu. This will bring the list of active applications. You can force quit all except Finder and then shutdown if any of them is preventing you from Shutdown.
If you see those opened apps on dock or bottom toolbar, then you can close them this way. Right click or hold down control key and click on opened apps and click Quit. Then you can shut down.
If you have any questions or would like any additional information, please don't hesitate to ask