The Dock in OS X serves as a launcher for commonly-accessed programs, as well as folders and documents that you drag there, and as an indicator of running programs. In addition, it holds the trash and the smiling Finder icon since this program as the default file browser is not usually quit.
Since the Dock is regularly accessed, it is always present in some form on the edge of the screen; however, while Apple by default places it at the bottom of your display, there are several options available to you for its position.
The first options are those Apple gives you by default, which are available in the Dock's system preferences pane, and are to position at the left, right, or bottom of your display. However, while these are present in System Preferences, there are two alternative ways of accessing these settings.
The first alternative is to right-click the separator bar on the Dock (between the applications and the documents and folders areas), and choose the position from the contextual menu that appears. The second is to simply shift-drag the Dock to the desired location on screen. To do this, simply hold the Shift key, and then click and drag the Dock by its separator bar to the right, left, or bottom of the display.
Beyond the adjustments that Apple allows through its settings interfaces, the Dock does support other features which may help you further customize the Dock's position, and place it not only at the bottom or to either side of your display, but also to the respective corners of any side.
These options can be set by using the "defaults" Terminal command, in the following manner:
defaults write com.apple.Dock orientation -string POSITION
In this command, replacing POSITION with either "left," "right," or "bottom" will place it on the respective edge of the display. This option is fairly redundant with the built-in options for setting the Dock's position, so it may not be all too useful; however, a second defaults command will allow you to pin the Dock to the various corners of the display, by specifying it be pinned to the start, middle, or end of the display edge. To set this, you will need to run the "defaults" command in the following manner:
defaults write com.apple.Dock pinning -string POSITION
In this command, replace POSITION with "start" to nest the Dock to the left of the bottom edge, or to the top of either side of the display; replace it with "end" to reverse this and place to the right of the bottom edge, or to the bottom corner of either side. To undo these modifications, simply use "middle" as the pinning location.
When finished with running either of these commands, you will need to re-launch the Dock in order for them to take effect, which can be done by restarting your system, logging out and back into your account, using Activity Monitor to force-quit the Dock process for your account, or by running the following command in the Terminal: