Kernel Exclusively 64-Bit
"Starting with Mountain Lion, OS X exclusively uses a 64-bit kernel, but it continues to run both 32-bit and 64-bit applications (OS X Mountain Lion Core Technologies Overview)"
Why this is a good thing:
There are two reasons this is a good thing. The first is simple: 64-bit computing is necessary if you want one of the programs on your computer to have access to more than 4GB of RAM. Second, there are some speed boosts associated with running in 64-bit mode. The Intel processors that power Macs have built-in math routines that operate more efficiently in 64-bit mode, processing tasks in fewer steps. That means that certain math-intensive tasks will see a speed boost under Snow Leopard’s 64-bit applications (Macworld).
However, having a 64-bit kernel does have its downsides: some older 64-bit computers like the iMac (pre-Mid-2007), Macbook Pro (pre-Mid-2007), Mac Pro (pre-2008), and others are not able to run Mountain Lion. This has to do with the fact that now with Mountain Lion, Macs can only boot into 64-bit mode, but prior to that, Macs could boot into both 32 and 64-bit mode, so older Macs were able to boot into 64-bit mode, but their EFI firmware was 32-bit and therefore can only interface with a 32-bit kernel.