The Dock serves a couple of purposes. The first is that it acts as a way to quickly launch commonly used applications. Just click on an application and it either starts up (if it isn't already running), or the Mac switches to it (if it is). An application that is running (also known as an active application) will display a faint glow below the Docks application icon.
The majority of the Dock is taken up with applications. If you look closely, youll see a faint line near the right side of the Dock. The area to the left of that line is reserved for applications.
You can both add and remove applications from this area. To remove an application, just hold down the Option key on your keyboard (its two keys to the left or to the right of the spacebar) and drag the application out of the Dock. Its icon will disappear in a puff of virtual smoke. Dont worry, if you do this you havent deleted the application from your Mac. Rather, youve just removed from the Dock the icon that represents that application. Basically you've removed the shortcut to the application, but the program still exists in the Mac's Applications folder.
When you launch an application that isnt in the Dock, its icon will also appear in the Dock and have that faint glow beneath it indicating that its running. When you quit that application, it will disappear from the Dock.
If youd like to add that or another application to the Dock in a more permanent way, just select it (from the Applications folder, for example) and drag it into this applications area. Its icon will appear where you place it and other icons will shift out of the way.
The Dock can also alert you to things that require your attention. For example, if iTunes cant find a track youve asked it to play and iTunes isnt the application youre currently working with, the iTunes icon may bounce up and down in the Dock so that youll switch to it to learn about the problem its having. Additionally, some applications will show badgesred indicators planted on the applications icon. Mail, for instance, will display a badge indicating the number of messages its received that you havent yet read.
And you can access some settings for active applications by clicking and holding on an applications icon. For instance, if iTunes is running, you can click on its Dock icon and rate the currently playing song, pause playback, play the next or previous tracks, or shuffle songs. When Mail is active, you can ask it to retrieve new messages or you can choose to compose a new message.