Overview
In NeXTSTEP and OpenStep, the Dock is an application launcher that holds icons for frequently used programs. The icon for the Workspace Manager and the Recycler are always visible. The Dock indicates if a program is not running by showing an ellipsis below its icon. If the program is running, there isn't an ellipsis on the icon. In OS X, running applications were identified by a blue luminous dot, though this is turned off by default in Lion.
In OS X, however, the Dock is used as a repository for any program or file in the operating system. It can hold any number of items and resizes them dynamically to fit while using magnification to clarify smaller resized items. By default, it appears on the bottom edge of the screen, but it can also instead be placed on the left or right edges of the screen if the user wishes. Applications that do not normally keep icons in the Dock will still appear there when running and remain until they are quit. These features are unlike those of the dock in the NeXT operating systems where the capacity of the Dock is dependent on display resolution. This may be an attempt to recover some Shelf functionality since OS X inherits no other such technology from NeXTSTEP. (Minimal Shelf functionality has been implemented in the Finder.)
The changes to the dock bring its functionality also close to that of Apple's Newton OS Button Bar, as found in the MessagePad 2x00 series and the likes. Applications could be dragged in and out of the Extras Drawer, a Finder-like app, onto the bar. Also, when the screen was put into landscape mode, the user could choose to position the Button Bar at the right or left side of the screen, just like the Dock in OS X.
The OS X Dock also has extended menus that control applications without making them visible on screen. On most applications it has simple options such as Quit, Keep In Dock, Remove From Dock, and other options, though some applications use these menus for other purposes, such as iTunes, which uses this menu as a way for a user to control certain playback options. Other Applications include changing the status of an online alias (MSN, AIM/iChat etc.) or automatically saving the changes that have been made in a document (There is no current application with this feature made available for OS X). Docklings (in OS X 10.4 or earlier) can also be opened by using the right-mouse button, if the mouse has one, but most of the time either clicking and holding or control-click will bring the menu up.
In OS X Leopard, docklings were replaced by Stacks. Stacks "stack" files into a small organized folder on the Dock, and they can be opened by left-clicking. Stacks could be shown in three ways: a "fan", a "grid", or a "list", which is similar to docklings. In grid view, the folders in that stack can be opened directly in that stack without the need to open Finder.
The latest iteration of the dock, found in OS X Leopard, Snow Leopard, and Lion presents a 3D perspective instead of the traditional flat one, resembling Sun Microsystems' Project Looking Glass application dock.
Read more about this topic: Dock (OS X)