As with NEXTSTEP and OPENSTEP before it, Mac OS X features a user interface element known as the Dock. This is perhaps unsurprising, as it was directly derived from the original NeXT operating system, and shares a great deal behind the scenes with its predecessor. However, to users familiar with the classic Mac OS, it represents a significant departure from the traditional Macintosh way of doing things.
By default, the Dock sits anchored to the bottom of the user's screen, filled with a number of different icons. To the left of the Dock sit applications. Not every application on the system is present by default; a standard set is created with a user account, featuring a number of different applications Apple feel likely to be useful to a new user. These include iTunes, Apple's music player and store; Mail, the direct descendant of NEXTSTEP's Mail client; Sherlock, a file and web searching program; and QuickTime, Apple's audio and video player and creator. Of course, these applications differ between versions; Sherlock disappeared from the default Dock in Mac OS X 10.3 ("Panther"), while iTunes made its debut in 10.1 ("Puma").
Any application the user wishes to place in the Dock can simply be dragged down from a Finder window. This does not physically move the item; it still exists in the filesystem wherever it was before the drag. However, its icon now sits in the Dock where it was dragged to. The user can freely rearrange their applications, perhaps moving iTunes a little to the left. They can also remove an application by simply dragging it out of the Dock entirely, where it will vanish quite literally in a puff of smoke. A running application will always appear in the Dock while it is currently open; if the user chooses not to keep it in the Dock, it will fade out when the program closes. A running application is designated by an upwards-pointing triangle beneath their icon.
On the right-hand side of the Dock sit other icons; these represent files, folders and URLs the user wishes to keep there. A dividing line separates the applications area of the Dock from these. Should I want quick access, for example, to my home folder, I can simply drag it into this area, where it will happily rest. As well as icons dragged in by the user, all minimised windows will shrink down into this area. Some applications' windows, notably QuickTime and DVD Player, will keep playing the current video when minimised here, showing off Mac OS X's compositing engine.
Ever-present in the Dock is the Finder, the file browser the Macintosh has used since the very beginning, and the Trash. The Finder's smiling face icon will always sit to the far left of the applications side; likewise, the Trash holds firm on the very far right. Of all the icons in the Dock, these two are unique in that they cannot ever be moved from these locations - they are completely undraggable, and cannot be moved about nor removed from the Dock. One other 'unique' icon is Dashboard, which appears in Mac OS X 10.4 ("Tiger") and above. This icon, despite having a running triangle beneath it, can still be removed while open. Even if this is done, Dashboard is still accessible via keyboard shortcut or a hot corner.
Although appearing at the foot of the screen by default, the Dock can be 'pinned' to left or right sides of the display, though will always be centred. The Dock can also be hidden, sliding into display when the user hovers over where it would normally be. However, there are ways around the restrictions on location. There exist secret options available to manually change the Dock's location with greater control than the System Preferences allow, such as pinning to the far-left corner, or the top beneath the menu bar. While these can be changed by tinkering in the Terminal, they can also be modified through applications such as the very thorough system administration utility TinkerTool.
As well as sliding in- and -out of view when hidden, the Dock features one more piece of eye candy: magnification. Turning this on, either through System Preferences or in the Apple menu, makes icons grow and shrink in size as you hover over them; the current icon beneath the pointer will grow while all the others around it become progressively smaller, all changing size as you scroll over them. Since Dock icons can be a maximum of 128x128 pixels in size, this can lead to quite stunning rapid scaling of the bitmaps. The current size of the Dock's icons is user-adjustable by dragging the separator between left- and right-hand sides, allowing for very small or very large Docks; with magnification turned on, the user could have their Dock at its smallest setting only to zoom larger when hovered. (Thanks to C-Dawg for this salient point)