For years, most operating systems maintained two lists of programs. One listed unopened programs until you needed them, like the Start menu (Windows) or the Launcher (Mac OS 9). The other kept track of which programs were open at the moment for easy switching, like the taskbar (Windows) or the Application menu (Mac OS 9).
In OS X, Apple combined both functions into a single strip of icons called the Dock.
Apple’s thinking goes like this: Why must you know whether or not a program is already running? That’s the computer’s problem, not yours. In an ideal world, this distinction should be irrelevant. A program should appear when you click its icon, whether it’s open or not—just as on an iPhone or iPad.
“Which programs are open” already approaches unimportance in OS X, where sophisticated memory-management features make it hard to run out of memory. You can have dozens of programs open at once.
And that’s why the Dock combines the launcher and status functions of a
modern operating system. Only a tiny white reflective dash beneath a
program’s icon tells you that it’s open—and you can even hide that, if
you want. (Choose 
→Dock→Dock Preferences, and turn off “Show indicator
lights for open applications.”)
Apple has made it as easy as possible to learn to like the Dock. You can customize the thing to within an inch of its life, use it to control and manipulate windows in elaborate ways, or even get rid of it completely. This section explains everything you need to know.
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