Add to the Dock
Any icon you add to the Dock is simply a shortcut and is visible as a large button.   Only a single click is required to launch or access the item.  You can add numerous types of items to the Dock such as applications, folders, documents, and disks.  It's the ideal location to place your most frequently used items.

The Dock is divided into two unequal halves, partitioned by a vertical bar known as the divider. The left side is for application icons whereas the right side is for the Trash icon and the other addable items listed above.


You can easily rearrange the position of items in the Dock. All you do is click and drag the icon to the desired location. Note however that applications must stay on the left.

Just know that when you place an item in the Dock you are just placing an icon to represent that item.  So when an application is placed on the left side of the divider Mac OS X puts a copy of the applications icon in the Dock.  The application icon still resides in the Mac OS X application folder.  You can verify this by navigating to the Mac OS X Applications folder.  Select the icon for DVD Player.  Slowly begin to drag the icon towards the Dock.

 

You'll see that the icon along with the name will be shaded in light gray as you drag it.  Continue to drag it towards the left side of the Dock.  As you keep your mouse button pressed just hover it over the Dock between two existing icons.

You'll immediately see those two icons move apart to make room for the icon.  Keep the mouse button pressed and move the DVD Player icon to the left and the right to see similar behavior for the other icons.

After you have decided on the location where you would like to place the icon release the mouse button.  You will that the DVD Player icon was added and that the existing DVD Player icon is still present in the Applications folder.

 

Add Other Items to the Dock
If you adding something other than an application to the Dock it must be placed to the right of the divider.  A great item to add to the Dock is the Applications folder.  Other popular items to add are disks.  For example, the external disk is shown below:

As you can see Mac OS X will display folders and in the same hierarchical manner they exist on the system as indicated by the arrows.  The disk has three folders as shown in the left box.  Five folders exist underneath the Backups folde.  If I wanted to navigate further I would then choose one of those five to display their contents.

 

Try addiing items that you feel are useful such as shorcuts to URLs and images.  It's totally up to you!