Snow Leopard, as the name implies, builds directly on the solid base of Mac OS X 10.5 Leopard, and focusses on reducing the operating system's footprint. On top of that, a number of new features and low-level frameworks are implemented as well, such as Grand Central and OpenCL. This is also the first release of Mac OS X which no longer works on PowerPC-based Macintosh machines, and has the option of running in full 64bit goodness, including applications like the Finder and iTunes.
Alongside the client variant, the server version of Snow Leopard will also start shipping on August 28. Mac OS X 10.5 Leopard owners can buy the upgrade version of Snow Leopard for a mere 29 USD, while Tiger users will have to buy the box set (which includes iWork and iLife) for 169 USD. If you bought your Mac after June 8 2009 you are eligible for the Up to Date program, which means you'll get an upgrade copy of Snow Leopard for 9.95 USD.
You can place your pre-order in the Apple Store.