LaunchPad is an app launcher that resides in the Dock. From LaunchPad you can view all of your installed applications. You can search for an app and use the keyboard to launch it. You can also organize your apps by re-arranging them and placing them into folders to group them. LaunchPad is also an uninstaller, allowing you to remove apps that were installed by the Mac App Store.
In Mountain Lion you can now quickly search for apps by name in LaunchPad. This, in conjunction with the keyboard shortcut for LaunchPad, can make it quick and easy to launch apps. Organizing your apps in LaunchPad is limited, but the search function makes it irrelevant as you no longer have to look for apps by icon.