Apple’s Mac OS X Dock is a bar of icons at the bottom or side of the Desktop that provides access to selected applications, displays applications that are currently running, holds windows in their minimized state, displays selected folders in Stacks, and displays an icon for the Trash.

By default a Dock positioned at the bottom of the screen will be displayed as a reflective 3D platform while a Dock on either lateral side of the screen will be displayed as a 2D box:

The display the Dock at the bottom of the screen as a 2D box, open a new Terminal window and enter these commands:

defaults write com.apple.dock no-glass -boolean YES
killall Dock

To revert the change, open a new Terminal window and enter these commands:

defaults write com.apple.dock no-glass -boolean NO
killall Dock

Related posts:

1. Mac OS X: Disabling bouncing Dock icons
2. Making hidden application icons transparent on Mac OS X
3. Displaying gradients behind Stack items on Mac OS X
4. Creating a Recent Items Stack on the Dock in Mac OS X
5. Showing hidden files and directories in the Finder in Mac OS X