This Apple knowledge-base article describes Rosetta and its use for Universal Applications.
macRumors forum thread on do I still need Rosetta?
And, from The basics you need to know about Snow Leopard
Snow Leopard does support applications that haven’t been
upgraded to run on Intel CPUs,
but you’ll have to install Apple’s Rosetta tool to run pre-Intel software on Intel hardware.
Apple seems to have deliberately removed Rosetta from Snow Leopard to goad users into abandoning their older Macs.
This is causing some fuss in the Mac community.
Including this local discussion --
Superuser: What is Rosetta and why doesn’t Snow Leopard include it by default?
From Mac OS X 10.6 Snow Leopard FAQ,
Do I have to have Intel or Universal Binary versions of all my software, or will my Mac OS X PowerPC-based software and Classic Mac OS software still run in Snow Leopard?
Like Leopard, Snow Leopard includes Rosetta, which makes it possible to run Mac OS X software written for the PowerPC on an Intel Mac. (Rosetta is now an optional install in Snow Leopard.)
As for Classic Mac OS software, you're mostly out of luck. The Classic Mode, found in Tiger and earlier versions of Mac OS X, disappeared in Leopard and has not returned in Snow Leopard. An emulator such as SheepSaver or Mini vMac would work, though it would be more work to set up than the (formerly) built-in Classic Mode.