Recently we have received a few questions from Windows users who have switched to the Mac OS and who have wondered about some behavior differences between OS X and Windows that can, at times, result in frustrating experiences. There are numerous Mac basics that are covered at Apple's support site, among other places, but one area of frustration that seems to be more common than others (even for relatively long-standing Mac users) is the difference between quitting an application and closing its windows.
MacFixIt reader "Tim" wrote in about this exact experience: when trying to install the latest version of Office on his new Mac the process became hung up when it requested he quit Safari. As part of the installation, Office will install Microsoft's Silverlight Web plug-in and so will require you to close your Web browsers (even though this technically is not needed to complete the installation). As PC users are accustomed to doing, Tim closed all Safari windows, but was stuck with Office still claiming he needed to quit Safari.
Unlike in Windows, when you press the red button on a Mac's window it closes the window only, and generally does not close the parent application for that window. In a few applications like Calculator, closing the window will close the application itself, but usually this is not the case. In the case of Safari and most other applications, when you click the red button the window closes but the Safari icon in the Dock has a small white dot under it that means the program is still open.
This behavior is one of the big differences between the Mac and Windows, and does cause some confusion for folks who have switched. In Windows each window represents a separate instance of an application, but in OS X each window is best seen as an open document for a single instance of one running application (this is true even for the Finder windows--once you close them all the Finder still remains as an active and open application).
To fully quit an application, you need to send it the Quit command by choosing this from the application menu. The Application menu will be the same name as the foremost application that is open, and is located immediately to the right of the Apple menu at the top left of your screen. If you click the Finder the application menu will change to be called Finder, or if you launch Safari or bring it forward to be the foremost application then the name of this menu will change to Safari, and likewise for any other application.
Another way to quit applications is to use the Command-Q keyboard shortcut when the application is in the forefront (its name is next to the Apple menu).
When you quit an application, all saved documents (or open windows that cannot be saved) will be closed and the application will be shut down. If a document contains unsaved changes then the window for that document will be brought to the front and you will be asked to save, not save, or cancel (which will halt the quit process). If you suspect an application has not quit, the best way to check whether or not it is still open is to see if a white dot is underneath it in Snow Leopard's Dock (in prior versions of OS X this dot is a black triangle).
While the dock will show running applications, it will also show those placed in the Dock that are not running. Therefore, another way to see if an application is running is to open the Force-Quit window by pressing Option-Command-Escape and checking the list of applications in the window, or by simply pressing Command-Tab to switch applications (though this can result in inadvertently switching focus to another application).
Using the application-switcher keyboard shortcuts, you can quickly quit applications provided they do not have any open documents with unsaved changes. Just press the Command key and then press the Tab key to bring up the switcher. Continue to tap the tab key until the switcher highlights the application you want to quit, and then immediately press "Q" (keep the Command key held down) and the program should be sent the signal to quit.