The Dock may be one of the handiest features introduced in OS X. By default, the Dock is located across the bottom of the screen, and is always in view. I find this convenient, because it provides quick access to my favorite applications.
However, some users (like my otherwise sensible wife) prefer to keep every available inch of screen real estate, well, available. To them, an always-visible Dock just gets in the way when they're not using it. No matter how wrong that view might be, Apple designed the Dock to be flexible. And who am I to argue with Apple (or my wife)?
You can easily change the Dock's settings, so that it only appears when you mouse over it.
Hide or Show the Dock
Click the ‘System Preferences’ icon in the Dock.
Click the ‘Dock’ icon in the Personal section of the System Preferences window.
Put a check mark in the ‘Automatically hide and show the Dock' box if you want the Dock to go away when you're not using it. Remove the check mark if you want the Dock to always be visible.
Close the Dock's preferences pane.
The Dock will now disappear when it's not in use. You can make it reappear as needed by moving your mouse cursor to the bottom of the screen, where the Dock normally resides. (Of course, if you've already moved the Dock to the left or right edge of the screen, as described in the Customize the Dock's Location Quick Tip, you'll need to mouse over the appropriate location to see the Dock.)