Many people using the Mac these day are recent converts from Windows, and may have developed an affinity for some of the interface features in Windows. One of these is the capability to maximize windows to full screen using the windows' maximize button; however, the analogous button in OS X performs a slightly different default function.
In Windows the maximize button offers one function, but Apple has reserved the green button for a variety of resizing features, calling it the Zoom button instead of the maximize button. Unfortunately this description is not always accurate, since the behavior of this button is rather inconsistent and depends on the specific program being used. As a result, even seasoned Mac users may be confused about what the Zoom button will actually do in a given program.
The best way to treat the green Zoom button in OS X is that it will in "some" way perform a resizing function on the current window. For the most part it will resize the window to best fit its contents, but may also change the type of window being displayed (ie, iTunes' mini player), switch between aspect ratios for media players, or even perform a full maximize function.
Despite the inconsistencies in the default behavior of the button, it still can be used to fairly regularly maximize Windows in OS X, similarly to how the analogous button works in Windows. To do this, just hold down the options key whenever you click the green button.
Personally, I feel OS X would benefit more by making this the default behavior, and then providing resizing and other "fitting" features if people hold the option key. We'll see what future versions of OS X will do, but for now and the foreseeable future, the green button performs an arbitrary zoom function.