In Mac OS X 10.10 (Yosemite), Apple changed the window interaction buttons. This change does not improve Mac OS X usability. This post explains why this is so.
Before Mac OS X 10.10 (Yosemite), Mac OS usually provided three buttons for window interactions:
The green button was used to “zoom” the window, meaning that the window would enlarge to display all the window content. Because the application would decide how to handle the content, the zoom behaves a bit inconsistent: sometimes it would make the window as large as the whole screen (maximizing it), sometimes the window would only enlarge to fill a certain amount of the screen.
Mac OS X also displayed a fourth button to make the window fullscreen, for applications that supported this mode:
But in the latest version of Mac OS X, 10.10 Yosemite, the situation has become an unfortunate mess for the long term Mac OS user.