The Dock is OS X's control center. You can drag application icons, documents, folders, and disk icons to the Dock, and it's where open windows in OS X-native applications go when you minimize them. You can set the size of the icons there, from tiny to huge. And you can turn on Dock magnification, so that even tiny icons balloon up when you pass the cursor over them. You can hide the Dock, having it show up only when you move the cursor to the bottom of the screen. But you can't turn it off completely.
The Dock is separated into two parts by a dividing line. Applications live on the left side; disks, folders and files on the right. This helps you figure out what's what on the Dock, but the ability to create custom groupings would be nice.
Left Dock The left side of the Dock is the functional equivalent of OS 9's application menu. Active applications appear on the Dock automatically, but if you drag an application's icon there manually it will remain on display, whether or not the program's running.
Right Dock Disks, folders, files, and URLs live on the right side of the Dock. You can drag icons representing any of these to the Dock to make them readily available. This is also where minimized windows appear (only from within OS X-native applications).
Launch Pad Running applications appear on the Dock with a small black triangle beneath them. The default Animate Opening Applications option in the Dock preferences pane makes icons bounce up and down while their respective applications are being launched. With animation turned off, the black triangle under the icon blinks while an application is launching.
Dock Extras In its default view, the Dock contains three special icons known as doc extras: Displays (not shown), Battery, and AirPort. The Battery icon shows up only if OS X is running on a PowerBook or iBook. If you're running on batteries and you move your mouse over it, the icon shows how much battery time you have left. If your PowerBook is plugged into an AC outlet, it shows you how long it will take your batteries to charge fully. The AirPort icon shows up only on AirPort-equipped Macs. It indicates how strong your AirPort signal is, lets you turn AirPort on or off, and provides a quick connection to AirPort networks. You use the Displays icon to select different screen resolutions and bit depths.