"Focus follows mouse" or "sloppy focus" is a feature common to X11 window managers on Linux/Unix, including GNOME, KDE, CDE, XFCE and window managers like Enlightenment, Fluxbox and Window Maker. It is also available via TweakUI on Windows. Some individual applications on OS X, like iTerm support it.
What is it? Simply put, the window where the mouse pointer is has focus, rather than having to click a window for it to gain focus.
Does the native GUI for OS X support this, with some hidden setting?