Let's say you launch an app that is not already positioned on your Dock. For example, you launch it from your Applications folder. Typically, it will plop itself onto your Dock - way over at the right end, assuming your preference is to have the Dock at the bottom of your screen. It will show up next to a small vertical line on the Dock's shelf.
In Lion and Mountain Lion, this line is very faint; almost invisible. It delineates the Dock area for apps (left side) and the area for documents, folders and the system trash can (right side). Plus, it leads you to some other goodies that I will mention a bit later.
The Dock has visual cues indicating running apps.
You will notice little blue "LED lights" underneath some app icons on the Dock. These "lights" indicate that the app associated with them are, in fact, launched and running. Again, as with that little line on the Dock, these are much more evident under Snow Leopard and older versions of (Mac) OS X.
The line on the Dock separates apps from files and folders.
You may be wondering why Apple is making these Dock indicators harder to see now in the "Lions." The thinking is that knowing whether an app is running or not is no longer useful or required.
Without going into much detail, it stems from the fact that, as with iOS apps, and because of the new Auto Save features that make up part of Apple's new Modern Document Model, the Mac system will quietly quit any background apps that have not been active for a given amount of time.
This is done in an effort to have OS X reclaim memory and operate more efficiently, while keeping your documents with all changes and versions always available to you. Of course, all this presupposes that the app in question supports this new document model.
In the illustrations above and below, there are three apps in the Dock that are currently running. You probably already know that while certain apps are running, you might not see them on the screen if they are "hidden" from view for any number of legitimate reasons controlled by you. This might explain the original intent of the little Dock "lights" - to visually check which apps are running even though you don't see any open application windows.
If you press-and-hold your mouse button or trackpad with the pointer directly over any of the app icons on the Dock, in a couple of seconds, a translucent black pop-up menu … well ... pops up! You can navigate the menu while moving the pointer arrow into the pop-up menu and to the desired menu item.
You can produce pop-up menus from the Dock items in order to access a number of options.
In the image above, you can see the resulting pop-up menu for the Messages app. Notice the Options menu item which extends a sub-menu. By selecting items in this sub-menu, you can enable (as indicated by a check-mark) and/or disable (no check-mark) certain options.
In this example, I have enabled two options. With the Keep In Dock option enabled, when Messages quits, and the little Dock "light" extinguishes, the app icon will remain on the Dock for easy access later. As shown, I am also about to enable the Open at Login option. This will have Messages launch automatically at startup or login.
As I alluded to already, you know which options are enabled if you see a checkmark next to them. These kinds of menu items are called "toggle switches"; you select them to turn them on, and you select them again to turn them off.
Just FYI, the Show in Finder option can be pretty handy. When you select it, the Mac Finder will open up the folder where the app is located and show it to you in case you need to move it or delete it.
Note that other app icons on the Dock may show different items you in the pop-up menu, but just about all apps offer the Options menu.
A final tip: remember that little line on the Dock that I mentioned above? The one that separates the apps from files and folders? Right-click (or Control-click) on the line to see some handy options related to your Dock without having to go to System Preferences to set the same options.
Right-clicking on the Dock's separator line lets you set other Dock options and preferences.
The Dock - you either love it, or you hate it! There is more to say when discussing your optimum interaction with the Dock. Hopefully, this article has shown you some new tricks and tips on some great Dock timesavers.