Mac OS X version 10.6 "Snow Leopard" is the seventh major release of Mac OS X, Apple's desktop and server operating system. Mac OS X (pronounced /mæk oʊ ɛs tɛn/) is a line of computer operating systems developed, marketed, and sold by Apple Inc., and since 2002 has been included with all new Macintosh computer systems.

Source: http://en.wikipedia.org/wiki/Mac_OS_X

Apple introduced many new features with each new release of OS X. One of the major differences between the previous versions of Mac OS and OS X was the addition of the Aqua GUI, a fluid graphical user interface with water-like elements. Furthermore, every window element, texts, graphics or widgets was drawn on-screen using the anti-aliasing technology.[41] ColorSync, a technology introduced many years before, was improved and built into the core drawing engine, to provide color matching for printing and multimedia professionals.[42] Also, drop shadows were added around windows and isolated text elements to provide a sense of depth. New interface elements were integrated, including sheets (document modal dialog boxes attached to specific windows) and drawers. According to Aaron Hillegass, some of these are the main reasons the user interface of the operating system looks "so much better" than other systems.[43] Despite this, Apple has continued to change aspects of the OS X appearance and design, particularly with many tweaks to the appearance of windows and the menu bar. The 10.0 "skin" looks considerably different from 10.3's, for instance. One example of a UI behavioral change is that previewed video and audio files no longer have progress bars in column view; instead, they have mouse-over start and stop buttons as of 10.5.