Under the hood, Mac OS X 10.6 Snow Leopard offers a 64-bit OS for faster processing and greater memory access, support for multicore processing via Grand Central Dispatch and OpenCL, and faster Java and QuickTime performance.
Do note that Snow Leopard does away with two old Apple technologies. It no longer supports the long-waning AppleTalk network protocol, and it won't run on PowerPC-based Macs. If you have PowerPC apps (such as the Microsoft Office Update utility), note they will run if you have Apple's Rosetta technology installed. If Snow Leopard may not install Rosetta automatically, you can install it using the Optional Packages installer on the Snow Leopard installation DVD.
The Exposé feature has been integrated into the Dock, so if you hold and click an app icon in the Dock, previews appear for each window, allowing you to switch among them and apply other options.
Mac OS X 10.6 Snow Leopard: The Dock's New Look
The Dock has a new look, with slicker menus. Common options are now in the Options submenu, making initial menus less cluttered.