This is the big shift many mac users have been waiting for--the new OS is based on a unix kernel (darwin), supports classic Macintosh applications (in other words, most of the stuff you've got already running under MacOS), new Carbon applications (which are written in a manner similar to classic Macintosh applications, but can take advantage of the benefits provided for unix, and boasts the new Cocoa framework to support OS X native apps. See the Core OS site for details. OS X also takes advantage of OpenGL, Quicktime, and the newly developed Quartz (related to Adobe's PDF format) for graphics. But the biggest change is in the user interface, named Aqua, that incorporates some features from the NeXT. Some people love it, some people hate it, take a look and what you think.