What used to take a separate app is now built in to OS X El Capitan. There is a setting that lets you hide the menu bar, just as you can hide the Dock. It remains hidden until you mouse over it, giving you a pristine desktop that lets you use every last inch, which is likely a particularly attractive option for those on compact MacBooks such as the and .
To hide the menu bar, head to System Preferences > General.
Next, check the box for Automatically hide and show the menu bar.
For more, get all you need to know about OS X El Capitan.