Just put the mouse over the buttons and you'll see symbols appear within the  buttons.   Hover over red and you'll see an 'x', hover over yellow to see a '-', hover over green to see a '+'.  You will begin to realize that out of the three, you will most often use the close button.

Close Button

To close a window, position your mouse pointer over the red button located at the upper-left corner of the window and click the mouse button.  Windows users are used to clicking the red 'X' in the upper right button in Microsoft Windows to quit an application.  In OS X however, clicking the left red button does not necessarily quit an application, in many cases it just closes the window for that application.

Some applications will quit when you close the window, some won't.  An ideal example to demonstrate this behavior would be a comparison of System Preferences and TextEdit.  System Preferences quits when you when you click the close button.  TextEdit however will continue to run when the close button is clicked.  You can verify this behavior using several different methods in Mac OS X for example, by looking under the application's icon in the Dock or by using Activity Monitor.

As a general rule, document-centric and/or applications that can have multiple windows open at any given time remain open when the window's close button is clicked. Single window applications on the other hand will quit.  Additional single window examples include Calculator, Dictionary, and DVD Player to name a few.

Minimize Button
You can minimize a window by clicking on the yellow button.  When you do this the window will find it's way to the right side of the Dock and will show up as a very small window.  The behavior is controlled by a Dock preference setting under System Preferences choices are "Genie Effect" (default setting) and "Scale Effect".  The example below shows the minimize of a Finder window to the Dock using the Genie Effect.  Note  how it looks in the Dock when fully minimized.

 

Once a window is minimized to the Dock just click on it to bring back in full view.

Zoom Button
The name "Zoom" is a bit misleading for the green button because the button not only zooms (or maximizes) but shrinks a window.    A click of the zoom button will make a window large enough so that you will be presented a view to show the relevant information for that window.  A subsequent click will return the window it's prior size.

The example that follows is of a Finder window.  Observe the first and second views closely.  Note that second is shown after clicking on the zoom button.  You can now see the additional column and sidebar listings.


Try to click on the zoom button in various applications to observe the behavior.


Updates

• March 17, 2009 - content revision, images updates and additions