Great Question, here is the answer: When you close the lid on your Macbook pro it goes into sleep mode. With OSX 10.9 Mavericks there is a mode that will allow your Macbook to still receive emails and alerts while it is sleeping. When you close the lid the Macbook immediately goes to sleep. When you leave your Macbook at idle it will fall asleep based on your preferences, which often times puts the screen to sleep before shutting down other internals. IMO it's better to just close the lid when your not using it, which also helps keep that pesky dust off the screen and keyboard.